Document podtx summarize in README

[frarredondo]

Summary

podtx summarize was shipped in df7409b (PR #33) but is missing from the README command table/docs. Its --help docstring and docstring are complete; only README coverage is missing.

Acceptance criteria

• README lists podtx summarize in the command table with usage (<path/to/episode.json>, --feed <slug> [--limit N], --all)
• README documents output sidecars (episode.summary.json / episode.summary.md: overview, key points, optional timestamped quotes)
• README documents backend/credential behavior: default fake (offline, no network); openrouter / opencode / lmstudio opt-in; keys via podtx auth set or env (OPENROUTER_API_KEY / OPENCODE_API_KEY) or --api-key

Related

Issue #8 (summarize feature, closed as shipped).