feat(continuity): wire §6.3 shared-docs staleness notice

MarkdownChanges was deleted as dead code, but the underlying spec
requirement wasn't abandoned — rebuilt it independently. Adds
continuity.ConventionsHash for AGENTS.md/CLAUDE.md/VOCAB.md, tracks a
per-session snapshot on herdr.Session, and adds
Coordinator.checkConventions (run every Monitor tick) which compares
each active session's snapshot against its project's base repo and
pushes an in-pane notice via a new herdr.ConventionsNotifier
capability when they drift.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W1rkJ2hBMybnJctPbcy4tT

internal/herdr/herdr.go

@@ -157,6 +157,12 @@ type Session struct {
 	// its §6.1 handoff (HandoffFile) — avoids re-sending the same prompt
 	// every tick while Release keeps waiting for the file to appear.
 	HandoffRequested bool `json:"handoff_requested,omitempty"`
+	// ConventionsHash is continuity.ConventionsHash of the project's shared
+	// *.md docs (AGENTS.md/CLAUDE.md/VOCAB.md) at the time this session was
+	// last notified of (or started with) their content — §6.3's staleness
+	// tracking lives at the orchestra layer, never trusted from the agent's
+	// cached view.
+	ConventionsHash string `json:"conventions_hash,omitempty"`
 }
 
 func (c *Client) Prompt(ctx context.Context, pane, text string, wait time.Duration) error {