feat(cli): add Codex handoff integration

[kleinpanic]

Summary

Adds Codex handoff support so existing Codex CLI sessions can be adopted into OpenACP and continued from a configured messaging adapter.

This is session-level continuation through the agent resume/session ID. It does not claim live mirroring of an already-running terminal process or replay of prior terminal scrollback.

Related Issues

None.

Type of Change

• New feature
• Bug fix
• Documentation
• Tests

Changes Made

• Adds a Codex integration spec to openacp integrate.
• Installs Codex handoff hooks and an openacp-handoff skill without replacing unrelated Codex hooks.
• Adds openacp adopt codex --tmux and --tmux-pane for explicit tmux-assisted discovery of Codex resume IDs.
• Uses a deterministic adopted-session title based on agent, workspace, and short external session ID.
• Documents Codex tmux adoption and clarifies that handoff is continuation, not terminal transcript mirroring.

Testing

• Ran pnpm build
• Ran pnpm test src/core/sessions/__tests__/adopted-session-title.test.ts src/cli/commands/__tests__/adopt-json.test.ts src/cli/__tests__/integrate.test.ts
• Ran git diff --check origin/main..HEAD
• Ran gitleaks git --log-opts origin/main..HEAD --redact --no-banner

Platform Impact

• CLI
• All / Core
• Telegram
• Discord
• Slack
• Web UI

Checklist

• Code follows the project's TypeScript conventions, including ESM .js imports
• Relevant documentation was updated
• Tests were added or updated
• pnpm build passes
• Changes are backward compatible with existing config and data formats

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