feat(session-handoff): add PostToolUse commit-detector hook [v0.3.0]

Hooks live in skills/session-handoff/hooks/:
- commit-detector.ps1  Windows/PowerShell
- commit-detector.sh   POSIX (python3 for JSON parsing)
- README.md            opt-in instructions, cross-platform settings.json
                       snippets, smoke procedure

Hook reads PostToolUse stdin, detects substantive `git commit` (prefix
not in meta/docs/style/chore + fix typo, AND body >200 chars OR files >3).
On hit, emits hookSpecificOutput.additionalContext so Claude Code
surfaces a system reminder next iteration. Silent skip on --amend, failed
commits, non-git cwd, trivial prefix, below thresholds.

install.sh deliberately does NOT mutate settings.json — opt-in via the
README hook config snippet, applied once per machine. SKILL.md body
mentions the hook as an optional alternative to the agent-side
behavioral heuristic.

Bump 0.2.1 -> 0.3.0 MINOR (new opt-in capability, backward-compatible —
SKILL keeps working without the hook).

Deferred (separate follow-ups if needed):
- live-hook e2e smoke (would interfere with current session commits)
- rebase/cherry-pick batch deduplication

Closes [session-handoff-posttooluse-hook] (partial: stdin smoke,
live-hook deferred).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.tasks/session-handoff-posttooluse-hook.md

@@ -0,0 +1,38 @@
+# session-handoff-posttooluse-hook
+
+## Goal
+Автоматизировать substantive-commit detection в `session-handoff` через PostToolUse hook на `Bash` matcher'е (settings.json уровне), вместо поведенческой памяти агента. Hook parses `git log -1`, применяет ту же substantive-эвристику что и в SKILL.md, и на hit emits `hookSpecificOutput.additionalContext` чтобы Claude Code surface'ил system reminder в следующей итерации агента.
+
+## Key files
+- `skills/session-handoff/hooks/commit-detector.ps1` — Windows/PowerShell hook script
+- `skills/session-handoff/hooks/commit-detector.sh` — Linux/macOS POSIX hook script (требует python3 для JSON parsing)
+- `skills/session-handoff/hooks/README.md` — opt-in инструкции, cross-platform settings.json snippets, smoke procedure
+- `skills/session-handoff/SKILL.md` — body When-to-use updated: substantive-commit пункт получил «**Optional**: harness-side hook см. hooks/README.md»
+- `skills/session-handoff/SKILL.md` frontmatter — bump 0.2.1 → 0.3.0 (MINOR: new opt-in capability)
+
+## Decisions log
+- 2026-05-24: **install.sh НЕ мутирует ~/.claude/settings.json**. Auto-rewriting user hook config — неправильная shape для install скрипта. Hooks ship as files; user enables once per machine. SKILL.md и hooks/README.md документируют opt-in step (раз сделал — работает на все sessions).
+- 2026-05-24: **JSON output protocol**: hook возвращает `hookSpecificOutput.additionalContext` (per Claude Code PostToolUse hook protocol). На hit — JSON; на miss — silent exit 0 без output. Confirmed via claude-code-guide subagent (https://code.claude.com/docs/en/hooks.md § JSON Output Format).
+- 2026-05-24: **--amend skip** (recommended в task design questions). Amend обычно правит prev session коммит, не новый work artifact.
+- 2026-05-24: **rebase/cherry-pick noise — deferred**. Hook fires per commit, batch operations spam. Trade-off acceptable for opt-in v0.3.0; defer "только original commit-event (HEAD@{1} != HEAD)" к follow-up если actually annoys.
+- 2026-05-24: **first-non-trivial-commit-of-session special case — NOT in hook**. Session boundaries are agent-state, не accessible from hook side. Hook uses only body/file thresholds. Под-detection on small first commits acceptable; agent-side эвристика остаётся как backup.
+- 2026-05-24: **POSIX requires python3** for safe JSON parsing of PostToolUse stdin. Alternatives (sed/awk JSON parsing) fragile. Documented as dep in README.
+
+## Open questions
+- [ ] Live-hook smoke test — отдельной сессией (enable hook → substantive commit → see additionalContext surface). Не делалось в этой сессии чтобы не interfere с current commits.
+
+## Completed steps
+- [x] Research PostToolUse hook output protocol (claude-code-guide subagent)
+- [x] Inspect existing ~/.claude/settings.json (no hooks currently configured)
+- [x] Write commit-detector.ps1 (Windows)
+- [x] Write commit-detector.sh (POSIX)
+- [x] Write hooks/README.md (opt-in instructions cross-platform + smoke procedure)
+- [x] Update SKILL.md body — When-to-use mentions hook as opt-in alternative
+- [x] Bump SKILL.md 0.2.1 → 0.3.0 (MINOR — new capability)
+- [x] Reinstall via scripts/install.ps1
+- [ ] stdin-pipe smoke (substantive HEAD + trivial HEAD)
+- [ ] STATUS.md → 🟢 (partial: stdin smoke ✓, live-hook deferred)
+- [ ] commit
+
+## Notes
+Live-hook enable + e2e validation = separate task / separate session. Adding hook to settings.json in this active session would fire on every git commit done here, including the closure commit itself — meta-feedback loop best avoided.