feat(codegen): optional local Claude Code agent via codegen-go

[teslashibe]

Summary

• Adds an opt-in local-execution path for the Claude Code CLI (and any other "prompt-on-stdin, edit-files-in-cwd" CLI like Codex / Aider / OpenHands / Cline / your-own) via the new teslashibe/codegen-go module.
• Complementary to — not a replacement for — the Anthropic Managed Agents path. Managed Agents stays the default for chat-style agent products.
• Reach for the new path when the agent needs to:
  • Run on this machine (filesystem access, on-prem, off-platform tools).
  • Drive a non-Anthropic CLI you've already integrated with.
  • Operate inside a long-running process under your direct control (e.g. an agent loop that opens PRs).

What's new

• New dependency github.com/teslashibe/codegen-go v0.1.1.
• backend/internal/codegen/codegen.go — ~100-LoC shim:
  • LoadFromEnv() — reads CODEGEN_* env vars and returns a configured Agent.
  • Re-exports Agent, Config, Result so callers stay on a single import.
• backend/internal/codegen/codegen_test.go — defaults / generic / missing-command / CSV-parser coverage.
• backend/cmd/codegen-demo/main.go — runnable proof:

  go run ./backend/cmd/codegen-demo "Summarise this directory in one paragraph."

• New CODEGEN_* block in backend/.env.example (commented out, opt-in).
• New "Local code-generation agent (optional)" section in README.md.

What's unchanged

• Managed Agents flow: cmd/provision, internal/agent, the SSE event shape, every API route — all identical.
• All existing tests pass (9 packages, zero failures).

Test plan

• cd backend && go vet ./...
• cd backend && go build ./...
• cd backend && go test ./... — 9 packages green, including new internal/codegen suite
• CI on this PR