Feature: explicit File Watching and AI Controlled build modes

[Unthred]

Summary

Add per-project build-control modes: File Watching (normal debounce auto-build) and AI Controlled (file watcher observes only; builds only via explicit /run/rebuild, /run/ship-check, or manual UI).

Why

Hybrid busy/idle + debounce still lets BuildMonitor schedule builds while an agent edits (pause/timeout races). Agents need a deterministic ownership model.

Acceptance

• Per-project persisted mode (file-watching / ai-controlled), default File Watching
• GET/POST /mode control-plane API
• Central BuildTriggerPolicy — AI Controlled never auto-builds from file changes (including busy timeout)
• Mode switch: FW→AI cancels pending file-triggered rebuild; AI→FW does not surprise-build
• Status card MODE row + clear AI wording
• Agent skill: set AI Controlled at task start; idle does not mean build
• Settings UI to change mode
• Unit + workflow tests; live verification
• Linked to Enhancement: show agent control-plane state in project status #57 / #57: Show agent control-plane state on project status cards #58 / UX: restructure project status card for at-a-glance state #60

Related: #57 #58 #60

[Unthred]

Implements deterministic build ownership for agent workflows; related to #57 / #58 / #60.