feat: add --branch/--prompt to spawn CLI + file-based prompt delivery

[AgentWrapper]

Summary

• Fixes prompt truncation bug where long composed prompts passed via -p get mangled by tmux/zsh (same root cause as orchestrator system prompt fix in feat: first-class orchestrator session + file-based system prompt #101)
• Adds promptFile to AgentLaunchConfig — spawn() writes prompts to {sessionId}-prompt.md files, and all agent plugins use $(cat ...) shell substitution
• Adds --branch and --prompt CLI options to ao spawn, enabling revival of sessions on existing branches: ao spawn project --branch feat/existing-branch

Test plan

• pnpm typecheck passes
• pnpm lint passes (0 errors)
• pnpm test passes (129/129)
• Manually verified: spawned 5 sessions with --branch for existing PRs on integrator project — all launched correctly with prompts delivered via file

🤖 Generated with Claude Code

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[AgentWrapper]

Addressing review comments

All 4 bugbot comments reference code from old commits that were removed during rebase. After the rebase onto main:

1. "Restored session PR field always null" — The restore() implementation was moved to PR feat: implement session restore for crashed/exited agents #104 (already merged). This PR no longer contains any restore code. Additionally, parsePrUrl() was extracted as a shared helper.

2. "Runtime leaked if metadata write fails in restore" — Same as above — restore() is on main via PR feat: implement session restore for crashed/exited agents #104 with proper error handling.

3. "Duplicated PR parsing logic" — Resolved by extracting a shared parsePrUrl() helper that's used by metadataToSession(). The restore code on main can also use it.

4. "Test mock restore returns undefined" — This PR no longer modifies api-routes.test.ts or the restore route. PR feat: implement session restore for crashed/exited agents #104 includes properly mocked restore tests.