Add podtx summarize for episode summaries and takeaways

[frarredondo]

Summary

Raw transcripts are long. Users often want a short summary, key points, and timestamped quotes without re-listening.

Motivation / evidence

Library already contains hour-long interviews (12k–16k+ words). Manual summarization per episode doesn’t scale.

Proposal

podtx summarize path/to/episode.json
podtx summarize --feed corecursive-coding-stories --limit 5

Produce a sidecar summary (e.g. .summary.md or JSON) with:

• short overview
• key points / takeaways
• optional notable quotes with timestamps

Use a pluggable backend (local model and/or API), configured explicitly—no silent external calls.

Out of scope

Full chat-with-podcast UI, automatic publishing to blogs/newsletters.

Acceptance criteria

• Can summarize from existing transcript JSON without re-running ASR
• Output format is stable and documented
• Backend/credentials behavior is explicit and offline-friendly by default where possible
• Docs / --help updated