feat: add mm-worktree-task — launch MiniMax worktree implementation sub-sessions

Wraps ask-minimax-run to run a MiniMax M3 implementation sub-session in an
isolated git worktree, with the prompt assembled from a planner work item
(cp_cli item + documents + parent/siblings). Supports --dryrun, --collect,
and --upload, per the pattern used for the 2026-09-10 seven-worktree batch
(see agent-runtimes memory/gotchas-minimax.md).

Claude-Session: https://claude.ai/code/session_015nXvGNNNixJPFwZKZHNyha

tests/fixtures/mm-worktree-task-docs/list.json

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+{"documents": [{"name": "notes.md", "updated_at": "2026-09-01T00:00:00Z"}, {"name": "old-notes.md", "updated_at": "2020-01-01T00:00:00Z"}]}

tests/fixtures/mm-worktree-task-docs/notes.md

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+Some supporting notes.

tests/fixtures/mm-worktree-task-docs/old-notes.md

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+Stale notes, should not be bundled under recent.

tests/fixtures/mm-worktree-task-item.json

@@ -0,0 +1,11 @@
+{
+  "id": "1f17f61e-317e-4e10-bcd6-bee6a3264379",
+  "title": "Wire up CI merge gate",
+  "label": "task",
+  "state": "open",
+  "flow_state": "ready",
+  "body": "Make the CI merge gate real: run tests on push to main, not just PRs.",
+  "tags": ["ci"],
+  "parent_id": null,
+  "related_items": []
+}