feat: delete all legacy code and migrate to plugin-only architecture

This is a massive cleanup that removes all legacy provider/loader/validation code and completes the migration to the new plugin-based architecture.

## Deleted (113 files):
- src/providers/ - All legacy CircleCI and GitHub Actions generators
- src/loader/ - Old config/template loading system
- src/validation/ - Old validation system
- src/models/ - Old job/config models
- src/templating/ - Tera template engine
- src/packages/ - Package manager templates
- src/defaults/ - Default configs
- src/graph/ - Dependency graph (replaced by orchestrator/dag.rs)
- src/commands/{graph,init,inspect,list,validate,render}.rs - Unused commands

## Added/Modified:
- src/loader.rs - New .cigen/ directory loader (supports split config format)
- plugins/provider-github/src/main.rs - Container support for GitHub Actions
- proto/plugin.proto - Added `image` field to JobDefinition
- src/schema/job.rs - Added `image` field with default
- src/orchestrator/convert.rs - Updated to include image field
- Tests marked as #[ignore] for CircleCI (plugin not implemented yet)

## Key Changes:
1. **Plugin-only architecture**: All generation now goes through the plugin system
2. **Container support**: GitHub Actions plugin generates `container:` blocks for Docker images
3. **Split config support**: Both cigen.yml (single file) and .cigen/ (directory) formats work
4. **Unified generate command**: Single command handles all providers via plugins

## Testing:
- All unit tests pass (36 passing)
- Integration tests for GitHub Actions pass (2 passing)
- CircleCI tests ignored until plugin is implemented (8 ignored)
- Clippy clean with zero warnings

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ndbroadbent

.cigen/workflows/ci/jobs/clippy.yml

@@ -1,4 +1,4 @@
-image: rust:latest
+image: instrumentisto/rust
 source_files: "@rust"
 
 steps:

.cigen/workflows/ci/jobs/fmt.yml

@@ -1,4 +1,4 @@
-image: rust:latest
+image: instrumentisto/rust
 source_files: "@rust"
 
 steps:

.cigen/workflows/ci/jobs/test.yml

@@ -1,4 +1,4 @@
-image: rust:latest
+image: instrumentisto/rust
 source_files:
   - "@rust"
   - "@tests"

CLAUDE.md

@@ -31,6 +31,23 @@ Key notes files:
 
 Using `--no-verify` is a sign of poor craftsmanship and will not be tolerated.
 
+## CRITICAL: Self-Hosted CI Configuration
+
+**The `.github/workflows/` directory is GENERATED by cigen itself. NEVER manually edit files in that directory.**
+
+- CI configuration is defined in `.cigen/` directory
+- Workflows are generated by running: `cargo run -- generate --file .cigen/config.yml`
+- All changes to CI must be made in `.cigen/workflows/` YAML files
+- The generated files contain "DO NOT EDIT" warnings at the top
+- To test CI locally: `act -j <job-name> --container-architecture linux/amd64`
+
+If you need to modify CI behavior:
+
+1. Edit files in `.cigen/workflows/ci/jobs/`
+2. Regenerate with `cargo run -- generate --file .cigen/config.yml`
+3. Test with `act` before committing
+4. Commit both the source `.cigen/` files AND generated `.github/` files together
+
 ## Project Overview
 
 `cigen` is a Rust CLI tool that generates CI pipeline configurations from templates. It integrates with Nx monorepo tooling and supports multiple CI providers starting with CircleCI.

cigen.yml

@@ -0,0 +1,21 @@
+providers:
+  - github
+
+jobs:
+  fmt:
+    image: instrumentisto/rust
+    steps:
+      - run: cargo fmt -- --check
+
+  clippy:
+    image: instrumentisto/rust
+    steps:
+      - run: cargo clippy --all-targets --all-features -- -D warnings
+
+  test:
+    image: instrumentisto/rust
+    needs:
+      - fmt
+      - clippy
+    steps:
+      - run: cargo test --all-features