Receipt 0.5.0 — session-health automation

Builds on 0.4.0's session health with automation, history, and a reviewer-facing note.

• receipt guard — a Claude Code hook entrypoint. Stays silent until a session crosses your gate, then prints the single most important move where the agent sees it. With --notify it exits 2 so Claude Code feeds the nudge back to the model — strongest on the PreCompact hook, right before lossy auto-compaction.
• receipt health --json / --quiet --gate — machine-readable output and severity exit codes (0 / 10 watch / 20 degrading / 30 critical) for hooks and CI.
• receipt health --all — scores every past session and learns your personal pattern ("you tend to drift around turn ~12; X% of sessions compacted too late").
• Context tax — shows how much of a session is just re-sending itself (the quadratic cost behind both rising spend and fading quality).
• PR-comment health note — a collapsed, reviewer-facing <details> block when the work ran under degrading conditions; silent otherwise; opt out with "health": false. It never claims the code is wrong — only points to where to look.

Honest constraint: the token-only ledger cannot detect redundant file reads, identical-command loops, or semantic issues (hallucinations, drift) — those need data Receipt deliberately never stores. Features only ever flag "conditions correlated with drift," documented in docs/SESSION-HEALTH.md.

Still deterministic and local. 104 tests, typecheck clean. MIT.