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>

skills/session-handoff/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: session-handoff
-version: 0.2.1
+version: 0.3.0
 description: "Sliding handoff between CC sessions via .tasks/NEXT_SESSION.md. Read on session start: orient agent, ask user before action. Write on session-end phrase or substantive commit. Session-end phrases: «завершаем сессию», «сворачиваемся», «закругляемся», «wrap up session», «end session», «we're done for now». Trigger-line in CLAUDE.md: `session handoff: read on start, write on end`. Skip task-zone phrases: «закрываем эту таску», «pause», «отбой», «разбегаемся»."
 ---
 
@@ -26,6 +26,7 @@ Forward-looking, не timeline: handoff = связка новых вещей к
   - prefix НЕ в (`meta:`|`docs:`|`style:`|`chore:`|`fix typo`)
   - AND (body length > 200 символов OR files changed > 3)
 - Плюс: **первый** non-trivial commit сессии — всегда триггерит, даже если ниже порога (старт работы = context shift).
+- **Optional**: substantive-commit detection может быть автоматизирован harness-side через PostToolUse hook — см. `hooks/README.md` для opt-in инструкций. С enabled hook'ом первая часть becomes deterministic (parser-side, не behavioral memory).
 
 **Skip (false-positive guards):**
 - «закрываем эту таску» — task close, не session. Это zone `using-tasks`.