talents: add __main__ guard so cortex subprocess actually runs

Cortex spawns talents via `python -m think.talents` (think/cortex.py:32
TALENT_EXECUTION_MODULE, :241 spawn args). Without an `if __name__ ==
"__main__":` guard, the module loaded, defined `main()`, and exited 0
without ever invoking it — so every talent run since the agents→talents
rename in 0050be8c (2026-04-17) silently failed with "Talent exited
with code 0 without finish event" and no sense.json (or any other
talent output) was written.

Confirmed by surveying /data/solstone/journal/talents/sense/*.jsonl:
50/50 most recent runs error, all runs prior to 2026-04-17T16:53 finish.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>