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/continuity/continuity.go

@@ -50,6 +50,31 @@ func TaskFileHash(root string) (string, error) {
 	return hex.EncodeToString(sum[:]), nil
 }
 
+// ConventionsFiles are the §6.3 shared docs ("Shared *.md ... all agents
+// contribute") whose staleness the orchestra is responsible for tracking —
+// never the agent, which only sees its own cached copy.
+var ConventionsFiles = []string{"AGENTS.md", "CLAUDE.md", "VOCAB.md"}
+
+// ConventionsHash hashes the concatenation of whichever ConventionsFiles
+// exist at root, in that fixed order, so the result changes iff any of their
+// contents change (a file appearing/disappearing also changes it, since a
+// length-prefixed marker precedes each file's bytes). A repo with none of
+// these files hashes to a stable, comparable empty-set value rather than
+// erroring — §6.3 is optional infrastructure, not every project uses it.
+func ConventionsHash(root string) (string, error) {
+	h := sha256.New()
+	for _, name := range ConventionsFiles {
+		b, err := os.ReadFile(filepath.Join(root, name))
+		if err != nil {
+			fmt.Fprintf(h, "%s:0\n", name)
+			continue
+		}
+		fmt.Fprintf(h, "%s:%d\n", name, len(b))
+		h.Write(b)
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}
+
 type Dirty struct {
 	Path   string `json:"path"`
 	SHA256 string `json:"sha256"`