refactor: consolidate agent env vars into config.AgentEnv

Create centralized AgentEnv function as single source of truth for all
agent environment variables. All agents now consistently receive:
- GT_ROLE, BD_ACTOR, GIT_AUTHOR_NAME (role identity)
- GT_ROOT, BEADS_DIR (workspace paths)
- GT_RIG, GT_POLECAT/GT_CREW (rig-specific identity)
- BEADS_AGENT_NAME, BEADS_NO_DAEMON (beads config)
- CLAUDE_CONFIG_DIR (optional account selection)

Remove RoleEnvVars in favor of AgentEnvSimple wrapper.
Remove IncludeBeadsEnv flag - beads env vars always included.
Update all manager and cmd call sites to use AgentEnv.

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

internal/witness/manager.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/steveyegge/gastown/internal/agent"
+	"github.com/steveyegge/gastown/internal/beads"
 	"github.com/steveyegge/gastown/internal/claude"
 	"github.com/steveyegge/gastown/internal/config"
 	"github.com/steveyegge/gastown/internal/constants"
@@ -164,8 +165,14 @@ func (m *Manager) Start(foreground bool) error {
 	}
 
 	// Set environment variables (non-fatal: session works without these)
-	// Use shared RoleEnvVars for consistency across all role startup paths
-	envVars := config.RoleEnvVars("witness", m.rig.Name, "")
+	// Use centralized AgentEnv for consistency across all role startup paths
+	townRoot := filepath.Dir(m.rig.Path)
+	envVars := config.AgentEnv(config.AgentEnvConfig{
+		Role:     "witness",
+		Rig:      m.rig.Name,
+		TownRoot: townRoot,
+		BeadsDir: beads.ResolveBeadsDir(m.rig.Path),
+	})
 	for k, v := range envVars {
 		_ = t.SetEnvironment(sessionID, k, v)
 	}