fix: harden exit summary (skip empty summaries, bound shutdown work)

[cokekitten]

Implements the two hardening items from the follow-up comment on #26. Independent of the configurability PR (which adds the opt-out/model knobs); both may touch the README env table and changelog, so whichever lands second gets a trivial rebase.

What

1. Don't persist all-"None." summaries

The summary prompt instructs the model to write None. under each heading when nothing is worth recording, but the write path only checked result.hasMessages && result.summary — so trivial sessions (>= EXIT_SUMMARY_MIN_MESSAGES messages, zero substance) got a boilerplate entry appended to the daily log, where it is re-injected at every session_start for the rest of the day and indexed by qmd. A new isExitSummaryEmpty() helper filters these out, completing the curated-write gate the code already intends (cf. the comment on EXIT_SUMMARY_MIN_MESSAGES and the earlier fix for failure-boilerplate).

2. Self-imposed timeout for shutdown summary work

Pi core awaits session_shutdown handlers with no timeout, and generateExitSummary() is only bounded by the provider's own timeout — a hanging provider could block quitting indefinitely even for users who want the summary. Summary generation is now raced against PI_MEMORY_EXIT_SUMMARY_TIMEOUT_MS (default 10s). On expiry nothing is persisted; the late result is dropped.

Both surface in the README env table, the changelog, and the memory_status configuration section.

Tests (TDD per AGENTS.md)

Baseline before edits: bun test test/unit.test.ts → 172 pass; tsc --noEmit and biome check clean.

New tests (added red, then green):

• isExitSummaryEmpty(): all-"None." blocks (incl. bullet/case/missing-period variants, headings-only) are empty; any real content (even text merely mentioning "None.") is kept
• getExitSummaryTimeoutMs(): env parsing, invalid/negative values fall back to the 10s default
• handler-level: with a never-resolving API-key lookup and PI_MEMORY_EXIT_SUMMARY_TIMEOUT_MS=50, session_shutdown returns promptly and writes nothing (without the race this test can only fail via bun's per-test timeout)

After: 177 pass, 0 fail; tsc --noEmit and biome check clean.

Refs #26