[codex] Implement ACP auth and read/write session modes

[nick-pape]

Summary

Implements the next minimum ACP surface slice for authentication and session modes.

Changes

• Advertises no ACP auth methods with authMethods: [].
• Rejects unsupported authenticate() calls instead of silently no-oping.
• Adds read and write ACP session modes, with write as the default.
• Persists session.mode_changed events and restores the latest mode during loadSession().
• Emits ACP current_mode_update when setSessionMode() changes a session mode.
• Filters known first-party mutating workspace tools from model turns in read mode.
• Updates README and API reports.

Validation

• pnpm test in packages/acp-agent
  • 4 test files passed
  • 26 tests passed
• pnpm build in packages/acp-agent
• node common/scripts/install-run-rush.js build --to @fledgling/acp-agent

Notes

Model selection remains separate work for #9. Permission prompts for writes/commands remain separate work for #13.

[Copilot]

Pull request overview

This PR expands the ACP agent surface area by explicitly advertising no authentication methods, adding read/write session modes (defaulting to write), persisting/restoring mode changes, and filtering mutating workspace tools from model turns while in read mode.

Changes:

• Advertise authMethods: [] and reject unsupported authenticate() calls.
• Add ACP session modes (read, write), persist session.mode_changed, restore mode during loadSession(), and emit current_mode_update.
• Filter known first-party mutating workspace tools from the toolset presented to the model in read mode, plus docs/API/test updates.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Documents lack of ACP auth methods and describes read/write session modes and their behavior.
packages/common/src/session-events.ts	Adds SessionModeChangedEvent to the persisted session event union.
packages/agent-core/src/agent.ts	Implements auth rejection, session mode state, persistence/restoration, current_mode_update, and tool filtering by mode.
packages/acp-agent/src/agent-prompt-cancellation.test.ts	Adds tests for authMethods, authenticate rejection, mode defaults/updates/persistence, and tool filtering behavior.
common/reviews/common.public.api.md	Updates the public API report to include SessionModeChangedEvent.
common/reviews/agent-core.public.api.md	Updates the public API report to reflect updated method signatures.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.