feat(tools): ToolExecutor registry, mock, and native tools (issue #18)

[leo-aa88]

Summary

Implements the ToolExecutor boundary from design doc §12.2 G with a registry that parses workflow uses strings (tool.<name>.<operation>), a mock executor for tests, and native built-ins (echo, identity). Responses follow §13.2 (output map + meta with durationMs / costUsd placeholders).

API

Types (types.go)

• ToolExecutor — Call(ctx, ToolCallRequest) (ToolCallResponse, error)
• ToolCallRequest — Uses, With map[string]any
• ToolCallResponse — Output map[string]any, Meta ToolCallMeta

Registry (registry.go)

• ParseUses("tool.github.pull_request.get") → github, pull_request.get
• NewRegistry(graph) — looks up graph.Tools[name] and routes by ToolSpec.Type:
  • native → internal/tools/native registry (echo, identity)
  • mock → optional Registry.Mock ToolExecutor, else canned JSON {"message":"mock",...}
• Unsupported types return a clear error (MCP/HTTP deferred per issue scope)

Native (native/registry.go)

• echo — returns {"echo": <shallow copy of with>}
• identity — returns {"value": with["value"], "ok": <present>}

Errors (errors.go)

• *UnknownOperationError — Tool, Operation fields; returned when a native tool does not implement the operation (errors.As-friendly)

Mock (mock.go)

• MockExecutor — fixed Resp / Err or custom Fn

Normalize (normalize.go)

• Ensures non-nil Output; fills DurationMs from wall clock if still zero

Tests

• Native echo succeeds — tool.demo.echo with payload round-trips under output.echo
• Unknown operation — tool.demo.no_such_op → errors.As(..., *UnknownOperationError) with correct tool/operation
• ParseUses design-doc style path; MockExecutor isolation

Verification

• make fmt (go fmt ./...)
• make test (go test ./... -race)
• make vet

Closes #18

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