feat(witness): add --env flag for environment variable overrides

Extends the --agent flag with a more general --env flag that allows
setting arbitrary environment variables when starting a witness.

Precedence (highest to lowest):
1. CLI --env overrides
2. Role bead env_vars
3. config.AgentEnv() defaults

Examples:
  gt witness start greenplace --env ANTHROPIC_MODEL=claude-3-haiku
  gt witness restart greenplace --env DEBUG=1 --env VERBOSE=true

Co-authored-by: joshuavial <git@codewithjv.com>

internal/cmd/start.go

@@ -235,7 +235,7 @@ func startRigAgents(t *tmux.Tmux, townRoot string) {
 			fmt.Printf("  %s %s witness already running\n", style.Dim.Render("○"), r.Name)
 		} else {
 			witMgr := witness.NewManager(r)
-			if err := witMgr.Start(false, ""); err != nil {
+			if err := witMgr.Start(false, "", nil); err != nil {
 				if err == witness.ErrAlreadyRunning {
 					fmt.Printf("  %s %s witness already running\n", style.Dim.Render("○"), r.Name)
 				} else {