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/daemon/daemon.go

@@ -404,7 +404,7 @@ func (d *Daemon) ensureWitnessRunning(rigName string) {
 	}
 	mgr := witness.NewManager(r)
 
-	if err := mgr.Start(false, ""); err != nil {
+	if err := mgr.Start(false, "", nil); err != nil {
 		if err == witness.ErrAlreadyRunning {
 			// Already running - nothing to do
 			return