feat(executor): Add parallel tool executor and dev agent

[ATMackay]

Summary

• executor/ — Custom agent.Agent that bypasses ADK's sequential llmagent loop. Injects *genai.FunctionDeclarations via genai.GenerateContentConfig.Tools, calls model.GenerateContent directly, and fans out every tool call batch from a single LLM response as goroutines (sync.WaitGroup). Each dispatch logs tool start/done with per-call timing.
• devtools/ — 9 dev-focused tools, all path-resolved against an explicit workDir: bash (200 KB sandwich truncation, process-group kill via Setpgid+WaitDelay for reliable timeouts), read_file, write_file, edit_file (uniqueness guard + replace_all), list_dir, glob, search_files (with context lines), git_status/diff/commit/push/branch, spawn_agent (runs agent run dev as subprocess).
• agents/dev/ — NewDev() wires executor + devtools with a TDD-first system prompt (write failing tests first, verify build after every edit, conventional commits).
• cmd/dev.go — agent run dev --task "..." --work-dir . subcommand.
• pipeline/ — Adds "dev" stage type to runStage() switch. StageConfig gains an optional task field for dev stage instructions.

Key design decisions

Why bypass llmagent? ADK's base_flow.go iterates function calls sequentially in a plain for loop. Parallel execution requires owning that loop. agent.New(agent.Config{Run: customFn}) provides the hook without rip-and-replace of existing agents.

Reliable bash timeouts: /bin/sh -c cmd forks child processes that inherit the pipe. Killing only the shell leaves children holding the pipe open, blocking cmd.Wait() indefinitely. Fix: SysProcAttr{Setpgid: true} + cmd.Cancel kills the entire process group; cmd.WaitDelay caps any remaining drain time.

Test plan

• go build ./... — clean compile, no import cycles
• go test -race ./... — all tests pass including new:
  • executor/executor_test.go — parallel dispatch timing, no data race, unknown tool error, response construction
  • devtools/bash_test.go — sandwich truncation, timeout (now ~2s not 60s), exit code capture, workDir
  • devtools/edit_test.go — basic replacement, not-found error, uniqueness guard, replace_all
  • agents/dev/config_test.go — Validate() with/without WorkDir
• agent run dev --help — flags and description render correctly

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