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

specs/mm-worktree-task.spec.md

@@ -28,7 +28,8 @@ mm-worktree-task <item-uuid> [OPTIONS]
 | `--timeout <secs>` | `1800` | Max seconds the sub-session may run (`timeout <secs>` wrapper). |
 | `--context <name>` | resolved (see §0) | `cp-cli --context` used for every planner call. |
 | `--upload` | off | After the sub-session exits, upload its report as a document on the item and patch `flow_state` to `impl-complete`. Implemented only in combination with `--collect` (see Behaviour §6) since launch is detached. |
-| `--collect` | off | Don't launch. Print `git log --oneline main..HEAD`, `git diff --stat main...HEAD`, and the report file for an already-run slug's worktree. |
+| `--base <ref>` | `main` | Branch or commit the worktree is created from, and the ref `--collect` logs/diffs against. Needed when work lives on a long-running branch, or when one agent's branch must build on another's (e.g. an implementation agent starting from a test agent's `mm/<slug>` branch). |
+| `--collect` | off | Don't launch. Print `git log --oneline <base>..HEAD`, `git diff --stat <base>...HEAD`, and the report file for an already-run slug's worktree. |
 | `--dryrun`, `-n` | off | Assemble everything, print the bundle manifest, the full prompt, and the exact command that would run. Create nothing: no worktree, no branch, no launch, no bundle/prompt files left outside the report dir's dryrun preview. |
 | `--help`, `-h` | — | Show usage and exit 0. |
 
@@ -65,7 +66,7 @@ mm-worktree-task <item-uuid> [OPTIONS]
    - Fetch sibling tasks under the same parent: `planner items list <project> --parent <parent_id> --json` if `parent_id` is set (best-effort; skip silently if the command errors, e.g. flag unsupported for the active project).
    - Fetching uses the operator's own `cp_cli` auth — the sub-session itself never calls `cp_cli`.
 5. Write `<report-dir>/<slug>-bundle.md` with `## ` sections: `## Item`, `## Parent` (if any), `## Blocked By` (if any), `## Siblings` (if any), `## Documents` (full name list, marking which are bundled below), then one `## Document: <name>` section per bundled document with its body. In `--dryrun`, the bundle is still assembled and its manifest (section list + doc names) is printed, but the file is not written to disk — instead its path is printed as the path that would be written.
-6. Create the worktree (skipped entirely under `--dryrun` and `--collect`): `git worktree add <repo>/.claude/worktrees/mm-<slug> -b mm/<slug> main`. Fail clearly (exit 1) if the branch or worktree directory already exists.
+6. Create the worktree (skipped entirely under `--dryrun` and `--collect`): `git worktree add <repo>/.claude/worktrees/mm-<slug> -b mm/<slug> <base>` (`--base`, default `main`). Fail clearly (exit 1) if the branch or worktree directory already exists.
 7. Assemble the prompt file `<report-dir>/<slug>-prompt.txt`:
    - Fixed preamble: worktree path and branch; never `cd` elsewhere, never `git push`, never `git stash`, never touch `git config`; spec-driven (a behaviour change requires the matching `spec/*.md` requirement in the same commit); Python 3.12 / Pydantic v2 / `@pytest.mark.asyncio` only; do NOT run the full test suite, only the touched test files; don't fix test infrastructure, mock it; read specs and existing tests before editing, match existing names; keep the diff minimal; commit with `git add -A && git commit -m "<type>: <summary> (<uuid>)"` after verifying `git rev-parse --abbrev-ref HEAD` prints `mm/<slug>`; write the report to `<report-dir>/<slug>-result.md` with sections Summary / Files changed / Tests run / Decisions / Open questions; one-line stdout ack only.
    - Task frame: `Implement work item <uuid>: <title>. Read the bundle at <bundle-path> fully before editing. The item body is the requirement; documents are supporting context. Documents available on the item (not all bundled): <names>.`
@@ -81,7 +82,7 @@ mm-worktree-task <item-uuid> [OPTIONS]
    wrapped in a backgrounded subshell that, on exit, appends `<slug> rc=<rc> elapsed=<s>s` to `<report-dir>/launch.log`. The parent process returns immediately.
    Default allowlist: `Read,Write,Edit,Glob,Grep,Bash(pytest *),Bash(python3 *),Bash(ruff *),Bash(grep *),Bash(ls *),Bash(git add *),Bash(git commit *),Bash(git status*),Bash(git diff*),Bash(git rev-parse*),Bash(git log*)`.
 9. Print: report dir, prompt file path, stdout file path, launch.log path, and a one-line `poll with: tail -f <stdout file>` hint.
-10. `--collect`: instead of steps 6-9, for the given `<slug>`'s existing worktree, print `git -C <worktree> log --oneline main..HEAD`, `git -C <worktree> diff --stat main...HEAD`, and the contents of `<report-dir>/<slug>-result.md` if present. If `--upload` is also given and the report file exists:
+10. `--collect`: instead of steps 6-9, for the given `<slug>`'s existing worktree, print `git -C <worktree> log --oneline <base>..HEAD`, `git -C <worktree> diff --stat <base>...HEAD`, and the contents of `<report-dir>/<slug>-result.md` if present. If `--upload` is also given and the report file exists:
     - `python3 -m lib.cp_cli planner items documents put <uuid> mm-<slug>-<YYYY-MM-DD>.md --file <report>`
     - `python3 -m lib.cp_cli planner items patch <uuid> --flow-state impl-complete --description "mm-worktree-task: <slug> committed on mm/<slug>"`
     `--upload` without `--collect` is an error (exit 1): "`--upload` requires `--collect` (launch is detached; upload happens after the sub-session has produced a report)".