skills: pi-subagent, extracted from pi-review-panel, which now runs on it

The panel's mechanics -- headless pi runs that don't hang, answer
extraction, billed cost, retries, verifying what comes back -- are not
review-specific. pi-subagent covers one delegated task, one brief fanned out
to several models, or a job split into parallel parts; pi-review-panel keeps
only what a review adds.

run-agent.sh replaces run-member.sh and fixes what it got wrong:

- `timeout` is not on a Mac without coreutils, which failed every member
  with exit 127; it falls back to gtimeout, then perl's alarm (142 -> 124).
- pi exits 0 when the provider refuses the request, or the answer is empty;
  both are now exit 1, with stopReason and error in the cost file.
- a retry moves the earlier attempt to <name>.attempt<N>.* instead of
  overwriting its cost file.

New options: --tools (an allowlist; read,grep,find,ls is read-only by
construction), --cwd, --context-files, --brief, --deliver. status.sh
replaces the `pgrep -f run-member.sh` loop, which counted every panel on the
machine, and prints the cost table.

From dot request 20261021-114826-work-laptop-2b868c.

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