mm-worktree-task: --base flag + set -e safe returns (uncommitted WIP)

Committing work found uncommitted in the checkout: --base lets a unit be based
on a branch other than main (used throughout the M32 cut-over session for
u6-tests and hatchmate-migration bases), and the bare 'return' statements
become 'return 0' so they cannot abort under set -e when the preceding test is
false.

Per the project agent-safety rule: untracked/uncommitted work that matters gets
committed the day it is found, so local loss cannot take it.

Claude-Session: https://claude.ai/code/session_014KNyfHmHpMbdMrHZXDRkAp

.gitignore

@@ -2,3 +2,6 @@ local_secrets/
 *.swp
 *.swo
 *~
+
+# Agent worktrees — never committed.
+.claude/