feat(using-tasks): session lock guard v1.4.0

Adds `.tasks/.lock` awareness to the `using-tasks` skill:

- Session start new step 1: read `.tasks/.lock`; if type:"agent" with
  heartbeat ≤10 min → hard warning + require user confirmation; stale
  lock (TTL expired) → silently overwrite; absent/cleared → write
  type:"interactive" lock (120-min TTL).
- Session end new step 1: delete `.tasks/.lock` when type:"interactive".
- Structure section: `.lock` entry with gitignored callout.
- Rules bullet: "Honour `.tasks/.lock`".
- `.gitignore`: adds `.tasks/.lock` (ephemeral runtime state).
- dist/using-tasks.skill rebuilt.

Closes [using-tasks-session-lock].

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -83,3 +83,6 @@ coverage/
 
 # Per-machine Claude Code local settings — keep ignored despite !.claude/ above
 /.claude/settings.local.json
+
+# Runtime session lock — ephemeral, never committed (using-tasks skill)
+.tasks/.lock