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

scripts/mm-worktree-task

@@ -25,6 +25,8 @@ Arguments:
 
 Options:
   --slug <slug>           Worktree/branch suffix (default: first 8 chars of uuid)
+  --base <ref>            Branch/commit the worktree is created from and that
+                          --collect diffs against (default: main)
   --focus "<line>"        One-line focus appended to the task frame
   --brief <file>          Use this file as the task body instead of/in addition
                           to the fetched item
@@ -56,6 +58,7 @@ UUID=""
 SLUG=""
 FOCUS=""
 BRIEF=""
+BASE="main"
 DOCS="recent"
 EXTRA_TOOLS=""
 TIMEOUT=1800
@@ -69,6 +72,7 @@ while [[ $# -gt 0 ]]; do
     --slug) SLUG="$2"; shift 2 ;;
     --focus) FOCUS="$2"; shift 2 ;;
     --brief) BRIEF="$2"; shift 2 ;;
+    --base) BASE="$2"; shift 2 ;;
     --docs) DOCS="$2"; shift 2 ;;
     --tools) EXTRA_TOOLS="$2"; shift 2 ;;
     --timeout) TIMEOUT="$2"; shift 2 ;;
@@ -171,11 +175,11 @@ if [[ $COLLECT -eq 1 ]]; then
     err "no worktree found for slug '${SLUG}' (${WORKTREE})"
     exit 1
   fi
-  info "git log --oneline main..HEAD"
-  git -C "$WORKTREE" log --oneline main..HEAD || true
+  info "git log --oneline ${BASE}..HEAD"
+  git -C "$WORKTREE" log --oneline "${BASE}..HEAD" || true
   echo
-  info "git diff --stat main...HEAD"
-  git -C "$WORKTREE" diff --stat main...HEAD || true
+  info "git diff --stat ${BASE}...HEAD"
+  git -C "$WORKTREE" diff --stat "${BASE}...HEAD" || true
   echo
   if [[ -f "$RESULT_FILE" ]]; then
     info "report: $RESULT_FILE"
@@ -223,15 +227,15 @@ fetch_item() {
 }
 
 fetch_docs() {
-  [[ "$DOCS" == "none" ]] && return
+  [[ "$DOCS" == "none" ]] && return 0
   local list_json
   if [[ -n "${MM_WORKTREE_TASK_FAKE_DOCS_DIR:-}" ]]; then
-    [[ -f "${MM_WORKTREE_TASK_FAKE_DOCS_DIR}/list.json" ]] || return
+    [[ -f "${MM_WORKTREE_TASK_FAKE_DOCS_DIR}/list.json" ]] || return 0
     list_json="$(cat "${MM_WORKTREE_TASK_FAKE_DOCS_DIR}/list.json")"
   elif [[ -n "$UUID" ]]; then
     list_json="$(cp_cli planner items documents list "$UUID" --json 2>/dev/null || echo '{"documents":[]}')"
   else
-    return
+    return 0
   fi
 
   while IFS=$'\t' read -r name updated; do
@@ -368,6 +372,7 @@ if [[ $DRYRUN -eq 1 ]]; then
   echo "  cp-cli from:  ${CP_CLI_DIR:-<not found>}"
   echo "  cp-cli ctx:   ${CP_CONTEXT:-<cp-cli default>}"
   echo "  Slug:         ${SLUG}"
+  echo "  Base:         ${BASE}"
   echo "  UUID:         ${UUID:-(none, --brief only)}"
   echo "  Worktree:     ${WORKTREE}"
   echo "  Branch:       ${BRANCH}"
@@ -399,7 +404,7 @@ if [[ -d "$WORKTREE" ]] || git -C "$REPO_ROOT" show-ref --verify --quiet "refs/h
 fi
 
 info "creating worktree ${WORKTREE} on branch ${BRANCH}"
-git -C "$REPO_ROOT" worktree add "$WORKTREE" -b "$BRANCH" main
+git -C "$REPO_ROOT" worktree add "$WORKTREE" -b "$BRANCH" "$BASE"
 
 info "launching ask-minimax-run (detached, timeout ${TIMEOUT}s)"
 (