Add Stack Exchange as a source

[RichardBray]

The Stack Exchange API is public and keyless at low volume, with question/answer scores published. Useful 'what problems are people hitting' signal for technical topics, though site activity has declined.

Add scripts/stackexchange.ts using the /search/advanced endpoint filtered to the recency window, sorted by votes.

Constraint check: public API, keyless quota sufficient for one call per run. ✅

[RichardBray]

Note: the PR implementing this must also update the README - the source list at the top and the "Design principles" section both enumerate the current sources (Firecrawl, Hacker News, GitHub), and SKILL.md's "How it works" list does too.