fix(session-handoff): PowerShell hook body char count [v0.3.1]

`(& git log -1 --format='%b')` in PowerShell collapses multi-line subprocess
output into string[]. The threshold check used `$body.Length` which on a
string[] returns the line count, not char count — so the body>200 condition
was effectively comparing "more than 200 lines", which is much harder to
meet. Files-count saves it in practice for big commits, but small-file
big-message commits were under-detected.

Fix: join the array back into a single string with `-join "`n"` before
measuring length. Verified via stdin-pipe smoke against current HEAD:
body chars now report 1255 (vs 29 before — the line count).

POSIX `.sh` variant unaffected — `$()` collapses output and `${#var}` is
char count.

Bump 0.3.0 → 0.3.1 PATCH (bugfix, no behavior contract change).

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: 0.3.0
+version: 0.3.1
 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»."
 ---