Add TL;DR block to shared session pages

[Yevanchen]

Summary

• add a TL;DR block above shared session transcripts, with a local Query / Conclusion / Trace fallback
• add recall session share --tldr-file so agent workflows can inject a higher-quality markdown summary before deploy
• update the bundled Recall skill and share docs for the TL;DR publishing workflow

Tests

• cargo fmt --check
• git diff --check
• cargo test tldr --lib
• cargo test session_share_accepts_tldr_file --bin recall
• cargo test