feat: Filter agent session molecule noise from activity feed

Agent session molecules (gt-gastown-crew-joe, gt-gastown-witness, etc.)
update frequently for status tracking, creating noisy entries in the
activity feed. This change:

- Adds IsAgentSessionBead() to identify agent session beads
- Filters out "update" events for agent sessions from the event feed
- Still updates the agent tree so status is visible there
- Still shows create/complete/fail/delete events for agents

The filtering happens in addEvent() in the TUI feed model. Agent session
updates are identified by parsing the bead ID pattern and checking for
known agent roles (mayor, deacon, witness, refinery, crew, polecat).

Resolves: gt-sb6m4

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/beads/beads.go

@@ -822,3 +822,20 @@ func ParseAgentBeadID(id string) (rig, role, name string, ok bool) {
 		return "", "", "", false
 	}
 }
+
+// IsAgentSessionBead returns true if the bead ID represents an agent session molecule.
+// Agent session beads follow patterns like gt-mayor, gt-gastown-witness, gt-gastown-crew-joe.
+// These are used to track agent state and update frequently, which can create noise.
+func IsAgentSessionBead(beadID string) bool {
+	_, role, _, ok := ParseAgentBeadID(beadID)
+	if !ok {
+		return false
+	}
+	// Known agent roles
+	switch role {
+	case "mayor", "deacon", "witness", "refinery", "crew", "polecat":
+		return true
+	default:
+		return false
+	}
+}