fix(ralph-loop): isolate loop state to the session that started it

The state file lives at .claude/ralph-loop.local.md — project-scoped,
not session-scoped. The plugin's Stop hook fires in every Claude Code
session open in that project directory. So if session A starts a loop,
session B's Stop events also find the state file and block, feeding A's
prompt into B and consuming A's iteration budget.

This was masked by the transcript-parsing bug fixed in the previous
commit: that bug deleted the state file on the first Stop in any
session, so neither session looped. Fixing it exposed the leak.

Fix: setup writes CLAUDE_CODE_SESSION_ID into the frontmatter; the hook
compares against .session_id from its stdin JSON and exits silently on
mismatch. State files without session_id (written by old setup scripts)
fall through to preserve existing behavior.

plugins/ralph-loop/scripts/setup-ralph-loop.sh

@@ -141,6 +141,7 @@ cat > .claude/ralph-loop.local.md <<EOF
 ---
 active: true
 iteration: 1
+session_id: ${CLAUDE_CODE_SESSION_ID:-}
 max_iterations: $MAX_ITERATIONS
 completion_promise: $COMPLETION_PROMISE_YAML
 started_at: "$(date -u +%Y-%m-%dT%H:%M:%SZ)"