fix(daemon): stop session flood and add session GC

[doyled-it]

Problem

The session table grew without bound, spawning thousands of dashboard rows. Traced from a live ~/.wavecrest/state.db backup (62 MB / 3,346 sessions / 19,874 events). Two distinct defects:

1. Indiscriminate adoption — every claude process that fired a SessionStart hook was adopted as a persistent session row, including headless claude -p one-shots and scripted bursts. One worktree produced 3,267 rows at ~28/min, each a short invocation that started and stopped with zero tool use (1 PreToolUse / 1 PostToolUse across all of them).
2. No garbage collection — finished sessions and their events were never pruned, and sessions whose process died without a SessionEnd hook lingered as idle/working zombies forever.

Fix

• Defer wild adoption until a session shows real activity (first PreToolUse / PostToolUse). Start/stop-only invocations never create a row — this eliminates ~99% of the flood at the source. Planned (wavecrest-launched) sessions still bind on SessionStart, unchanged.
• Hourly GC (src/daemon/gc.ts): reaps non-terminal sessions idle > 24h to finished (clears zombies), and deletes finished sessions older than 7 days (FK cascade clears events/rollup/samples). Pinned sessions are exempt from both.

Tests

• New coverage for both GC queries, the GC pass, and the deferred-adoption contract.
• 14 targeted tests pass (stable across repeated runs).
• Full suite: 118 pass / 1 fail — the single failure (an SSE timeout in http.test.ts) is pre-existing and fails identically on main.
• No new type errors introduced (gc.ts is type-clean).

Out of scope (noticed separately)

• The usage-poller is failing to spawn its meta-claude (posix_spawnp failed, hundreds of consecutive failures) — opposite problem, worth a separate fix.