feat(using-tasks): session_break marker [v1.2.0]

Add a session_break marker so a task author can mark a task's
completion as a natural session boundary. After the task closes 🟢,
before tasks_claim_next, an autonomous agent prints the verbatim
SESSION BOUNDARY line and stops instead of chaining the next task.
Absent -> behaviour unchanged.

- STATUS.md format: optional **Session break:** field + new
  "### session_break marker" subsection (type bool|string, examples).
- Task completion step 6: after close, before claim-next, check the
  closed task's session_break; print boundary line + stop if present.
- Rules bullet "Honour session_break".
- Wiki concept page concepts/using-tasks-session-break.md + index + log.

MINOR bump: new optional capability, no existing behaviour changed.
Closes [session-break-using-tasks].

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.wiki/index.md

@@ -41,6 +41,7 @@ Catalog of all wiki pages. One line per page, organized by type. Updated on ever
 - [project-bootstrap-meta-isolation.md](concepts/project-bootstrap-meta-isolation.md) — project-bootstrap@1.11.0 — Step 1 ships meta-isolation block in `.gitignore` (`!.claude/`, `!.tasks/`, `!.wiki/`, ...) so own greenfield/upgrade projects re-enable agent meta-paths against global `core.excludesFile` cutter. Marker-based append-only on existing files; smoke-tested with negative control
 - [interns-grep-audit-design](concepts/interns-grep-audit-design.md) — interns-grep-audit-design
 - [session-handoff-skill-design.md](concepts/session-handoff-skill-design.md) — design rationale for the `session-handoff` skill (sliding overwrite into `.tasks/NEXT_SESSION.md`, phrase whitelist + substantive-commit heuristic, optional PostToolUse hook for harness-side determinism, orient+ask default, project scope, cluster 7/7 closure)
+- [using-tasks-session-break.md](concepts/using-tasks-session-break.md) — `using-tasks` v1.2.0 `session_break` marker: task-author-set boolean/string flag; after a task closes 🟢, before `tasks_claim_next`, an autonomous agent prints the verbatim SESSION BOUNDARY line and stops instead of chaining the next task. Absent → unchanged
 - [delegate-task-negative-trigger-fp.md](concepts/delegate-task-negative-trigger-fp.md) — `delegate-task` v0.2.1 FP fix: «создать задачу себе» stem-matched the «создать задачу на агента» positive trigger; abstract "does NOT apply when doing the work yourself" carve-out loses to literal stem-match under the 1%-rule → made the negative literal + routed (→ using-tasks). Verified pos 5/5, neg 4/5 (was 0/5)
 
 ## Packages