Auto-generate session summaries from context

[vidhartbhatia]

Auto-generate session summaries using session context (files edited, tools used, task description) when no manual summary is provided. Would improve the session activity log quality.

[vidhartbhatia]

Design RFC posted: #32 → docs/summary-inference-rfc.md

Recommends Option E (Hybrid): passive suggestion records emitted at session-end + new review_suggestions / accept_suggestion / dismiss_suggestion tools. Heuristic-first (zero deps), opt-in in v1.1.0, default-on in v2.0.0. PR is draft — review before merge.