Enhancement: Adopt Claude Code `monitors` manifest key for Beads-state file watcher

[cofin]

Background

Claude Code's plugin manifest (.claude-plugin/plugin.json) supports a monitors key that registers background monitors auto-armed at session start or on skill invocation. This is a real, documented field as of April 2026 (plugins-reference).

Proposed use for Flow

Wire a Beads-state monitor that:

• Watches .beads/state.json (or the active backend's state file) for changes during an active Claude session
• On change, automatically runs /flow-sync to keep spec.md aligned with Beads — eliminating the current MANDATORY-but-easily-forgotten manual sync step after every Beads state change
• No-ops when Flow is not active in the current project (no .agents/ directory)

Why now

The repo's CLAUDE.md and skill rules mark /flow-sync after Beads state changes as CRITICAL. Today this discipline depends on the agent (or user) remembering. A monitors entry would convert the discipline into a passive guarantee.

Out of scope for this issue

• Cross-host parity. Codex / OpenCode / Gemini do not currently expose an equivalent monitor primitive — this is Claude-only, evaluated as part of the broader April 2026 host-mandate audit (see commit history of fix/codex-marketplace branch).

Acceptance criteria

• .claude-plugin/plugin.json declares a monitors entry for the Beads state file
• Monitor handler runs /flow-sync (or the equivalent skill action) only when .agents/ exists in the project root
• Idempotent on rapid file changes (debounce or coalesce)
• Documented in README and skills/flow/references/setup.md
• Covered by a test that simulates a Beads state mutation and verifies sync invocation