Remove ACP and the interactive-only file watcher #190

Decision

The CLI is headless-only. Remove:

• src/acp/ (~1.9k lines) and the aictrl acp command, plus @agentclientprotocol/sdk;
• the file watcher (src/file/watcher.ts, FileWatcher.init, @parcel/watcher if unused elsewhere);
• AICTRL_CLIENT === "acp" branches across the code.

Start after #183, #162 and #144 land, since they edit the same files.

Rules

• Delete the code, its tests and any dependencies only it uses. The net diff should be strongly negative. No default-off switch and no opt-in to bring it back.
• Compatibility shims only where existing configs or stored data would otherwise break:
  • config keys still load as deprecated no-ops, with one warning;
  • old sessions still load;
  • removed commands fail with a clear "was removed" error.
• Tracked under the headless optimisation epic Epic: Optimise headless run execution #153.