Change parallel mode in my /factory skill so each pull request runs in its own Claude
session instead of as a subagent of the main session.

1. The main session still verifies and plans the whole batch once, with a collision table
   and a merge order, and I approve it once.
2. It then writes each PR's part of the plan, the collision table, and a pointer to the
   other PRs to a file that survives the main session ending (not a temp directory).
3. For each PR, it runs from the repo checkout:
   claude --bg -n "<ticket> <short title>" "/factory <ticket url> --plan <that file>"
   then lists the sessions it started and stops. It doesn't follow them afterwards, and it
   never reports "done" itself.
4. Teach /factory a new entry: when given --plan <file>, skip verify and plan, read the
   file, and run build through done for that one PR. Ask me directly when stuck.
5. Keep my limit of 2 running at once with two slot locks (mkdir slot-1 / slot-2). A
   session takes one before building and releases it when done. A session with no free slot
   waits for one. A slot is free once the session that holds it is no longer running.
6. Keep the browser and dev-database locks as they are.
