feat: Standardize agent bead naming to prefix-rig-role-name (gt-zvte2)

Implements canonical naming convention for agent bead IDs:
- Town-level: gt-mayor, gt-deacon (unchanged)
- Rig-level: gt-<rig>-witness, gt-<rig>-refinery (was gt-witness-<rig>)
- Named: gt-<rig>-crew-<name>, gt-<rig>-polecat-<name> (was gt-crew-<rig>-<name>)

Changes:
- Added AgentBeadID helper functions to internal/beads/beads.go
- Updated all ID generation call sites to use helpers
- Fixed session parsing in theme.go, statusline.go, agents.go
- Updated doctor check and fix to use canonical format
- Updated tests for new format

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

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

internal/daemon/daemon.go

@@ -12,6 +12,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/steveyegge/gastown/internal/beads"
 	"github.com/steveyegge/gastown/internal/constants"
 	"github.com/steveyegge/gastown/internal/keepalive"
 	"github.com/steveyegge/gastown/internal/polecat"
@@ -327,7 +328,7 @@ func (d *Daemon) ensureWitnessesRunning() {
 
 // ensureWitnessRunning ensures the witness for a specific rig is running.
 func (d *Daemon) ensureWitnessRunning(rigName string) {
-	agentID := "gt-witness-" + rigName
+	agentID := beads.WitnessBeadID(rigName)
 	sessionName := "gt-" + rigName + "-witness"
 
 	// Check agent bead state (ZFC: trust what agent reports)
@@ -374,7 +375,7 @@ func (d *Daemon) pokeWitnesses() {
 
 // pokeWitness sends a heartbeat to a specific rig's witness.
 func (d *Daemon) pokeWitness(rigName string) {
-	agentID := "gt-witness-" + rigName
+	agentID := beads.WitnessBeadID(rigName)
 	sessionName := "gt-" + rigName + "-witness"
 
 	// Check agent bead state (ZFC: trust what agent reports)