Document payload directory configuration options (#702)

Nightly reconciliation identified 3 undocumented features for large
payload storage. The code supported `--payload-dir` flag,
`MCP_GATEWAY_PAYLOAD_DIR` env var, and `payload_dir` config option, but
none were documented.

### Changes

- **README.md CLI flags**: Added `--payload-dir` flag to usage section
with default `/tmp/jq-payloads`
- **README.md environment variables**: Added `MCP_GATEWAY_PAYLOAD_DIR`
to optional variables table
- **config.example.toml**: Documented `payload_dir` gateway option with
session segmentation pattern

All three methods configure the same directory where large tool response
payloads are stored, organized as
`{payload_dir}/{sessionID}/{queryID}/payload.json`.

Implementation in `internal/cmd/flags_logging.go`,
`internal/config/config_core.go`, and `internal/config/config_stdin.go`
unchanged.

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<details>

<summary>Original prompt</summary>


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*This section details on the original issue you should resolve*

<issue_title>📚 Documentation Reconciliation Report -
2026-02-05</issue_title>
<issue_description>## Summary

Found **3** discrepancies between documentation and implementation
during nightly reconciliation check.

- Date: 2026-02-05
- Branch: main
- Status: Minor documentation gaps found

## Important Issues 🟡

Issues that are misleading but have workarounds:

### 1. Missing `--payload-dir` Flag in README Usage Section

**Location:** README.md, line 214-226 (Usage section)

**Problem:** The README's Usage section shows CLI flags but is missing
the `--payload-dir` flag that actually exists in the codebase.

**Actual Behavior:** The code defines `--payload-dir` flag in
`internal/cmd/flags_logging.go`:
```````go
cmd.Flags().StringVar(&payloadDir, "payload-dir", getDefaultPayloadDir(), 
    "Directory for storing large payload files (segmented by session ID)")
``````

**Impact:** Users reading the CLI help in the README won't know about
the `--payload-dir` flag and may not discover this feature for managing
large tool response payloads.

**Suggested Fix:** Add the missing flag to the Usage section in
README.md after line 220:
``````
      --payload-dir string  Directory for storing large payload files (segmented by session ID) (default "/tmp/jq-payloads")
```````

**Code Reference:** `internal/cmd/flags_logging.go:26-27`

## Minor Issues 🔵

Small inconsistencies or missing details:

### 1. Missing `MCP_GATEWAY_PAYLOAD_DIR` Environment Variable
Documentation

**Location:** README.md, line 232-263 (Environment Variables section)

**Problem:** The environment variable `MCP_GATEWAY_PAYLOAD_DIR` is not
documented in the Environment Variables section, even though it's used
in the code.

**Actual Behavior:** The code checks for `MCP_GATEWAY_PAYLOAD_DIR` in
`internal/cmd/flags_logging.go`:
``````go
func getDefaultPayloadDir() string {
    if envPayloadDir := os.Getenv("MCP_GATEWAY_PAYLOAD_DIR"); envPayloadDir != "" {
        return envPayloadDir
    }
    return defaultPayloadDir
}
``````

**Impact:** Users won't know they can set this environment variable to
customize the payload directory location, especially useful in
containerized environments.

**Suggested Fix:** Add to the "Optional (Non-Containerized Mode)" table
in the Environment Variables section:
``````markdown
| `MCP_GATEWAY_PAYLOAD_DIR` | Large payload storage directory (sets default for `--payload-dir` flag) | `/tmp/jq-payloads` |
``````

**Code Reference:** `internal/cmd/flags_logging.go:40-46`

### 2. Quick Start payload_dir Configuration Not Mentioned

**Location:** README.md, line 38-51 (Quick Start configuration example)

**Problem:** The Quick Start configuration example doesn't show the
optional `payload_dir` field in the gateway configuration, even though
it's supported.

**Actual Behavior:** The code supports `payload_dir` in both TOML and
JSON configurations:
- `internal/config/config_core.go`: `PayloadDir string` field in
`GatewayConfig`
- `internal/config/config_stdin.go`: `PayloadDir string` field in
`StdinGatewayConfig`

**Impact:** Users won't know they can configure the payload directory in
their configuration files.

**Suggested Fix:** Add a note or example showing the `payload_dir` field
can be set in gateway configuration. Note: `config.example.toml` already
documents this correctly, so this is just about the README examples.

**Code Reference:** 
- `internal/config/config_core.go:56`
- `internal/config/config_stdin.go:33`

## Documentation Completeness

### Missing Documentation
- `--payload-dir` CLI flag - implemented but missing from Usage section
- `MCP_GATEWAY_PAYLOAD_DIR` environment variable - implemented but not
in Environment Variables table

### Accurate Sections ✅

The following sections were verified to be accurate and match the
implementation:

- ✅ Docker Quick Start section - verified accurate
- ✅ TOML configuration format - verified accurate
- ✅ JSON stdin format - verified accurate  
- ✅ Configuration validation rules - verified accurate
- ✅ Variable expansion (`${VAR_NAME}`) - verified accurate
- ✅ Schema normalization - verified accurate
- ✅ Routing modes (routed/unified) - verified accurate
- ✅ Docker support - verified accurate
- ✅ All make targets in CONTRIBUTING.md - verified all exist and work
- ✅ Prerequisites (Go 1.25.0) - matches go.mod
- ✅ Binary name (awmg) - verified correct
- ✅ Authentication (MCP spec 7.1) - verified accurate
- ✅ All other documented environment variables - verified accurate

## Tested Commands

All commands from CONTRIBUTING.md were verified against the Makefile:

- ✅ `make build` - works as documented
- ✅ `make test` / `make test-unit` - works as documented
- ✅ `make test-integration` - works as documented
- ✅ `make test-all` - works as documented
- ✅ `make lint` - works as documented
- ✅ `make coverage` - works as documented
- ✅ `make install` - works as documented
- ✅ `make format` - works as documented
- ✅ `mak...

</details>


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- Fixes #701

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---

💡 You can make Copilot smarter by setting up custom instructions,
customizing its development environment and configuring Model Context
Protocol (MCP) servers. Learn more [Copilot coding agent
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Author: lpcox

README.md

@@ -218,6 +218,7 @@ Flags:
   -h, --help                help for awmg
   -l, --listen string       HTTP server listen address (default "127.0.0.1:3000")
       --log-dir string      Directory for log files (falls back to stdout if directory cannot be created) (default "/tmp/gh-aw/mcp-logs")
+      --payload-dir string  Directory for storing large payload files (segmented by session ID) (default "/tmp/jq-payloads")
       --routed              Run in routed mode (each backend at /mcp/<server>)
       --sequential-launch   Launch MCP servers sequentially during startup (parallel launch is default)
       --unified             Run in unified mode (all backends at /mcp)
@@ -254,6 +255,7 @@ When running locally (`run.sh`), these variables are optional (warnings shown if
 | `HOST` | Gateway bind address | `0.0.0.0` |
 | `MODE` | Gateway mode flag | `--routed` |
 | `MCP_GATEWAY_LOG_DIR` | Log file directory (sets default for `--log-dir` flag) | `/tmp/gh-aw/mcp-logs` |
+| `MCP_GATEWAY_PAYLOAD_DIR` | Large payload storage directory (sets default for `--payload-dir` flag) | `/tmp/jq-payloads` |
 
 ### Docker Configuration
 

config.example.toml

@@ -29,6 +29,11 @@ startup_timeout = 60
 # How long to wait for a tool call to complete before timing out
 tool_timeout = 120
 
+# Directory for storing large payload files (default: /tmp/jq-payloads)
+# Payloads are segmented by session ID: {payload_dir}/{sessionID}/{queryID}/payload.json
+# Can also be set via MCP_GATEWAY_PAYLOAD_DIR environment variable or --payload-dir flag
+# payload_dir = "/tmp/jq-payloads"
+
 # ============================================================================
 # MCP Server Configurations
 # ============================================================================