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/concepts/using-tasks-session-break.md

@@ -0,0 +1,52 @@
+---
+title: using-tasks session_break marker
+type: concept
+tags: [using-tasks, autonomous-runner, session-boundary]
+updated: 2026-06-09
+---
+
+# using-tasks `session_break` marker
+
+`using-tasks` v1.2.0 adds a `session_break` marker so a task author can mark a task's
+completion as a natural place to **stop**, rather than have an autonomous agent immediately
+chain into the next task via `tasks_claim_next`.
+
+## Problem
+
+An autonomous runner closes a task and, by default, claims the next one. There is no signal
+for "this is a good seam to end the session" — so unrelated tracks get welded into one
+ever-growing context, and the natural review/hand-off moment is skipped.
+
+## Design
+
+- **Marker:** `session_break` in the task's frontmatter (task-system delivery) or the
+  `**Session break:**` field in the task's STATUS.md block (local board mirror).
+- **Type:** boolean or string.
+  - `true` → pause after close; next track is "see STATUS.md".
+  - `"<hint>"` → pause after close; the hint names the recommended next track.
+- **Enforcement point:** `using-tasks` → Task completion, **step 6** — *after* the task is
+  🟢 and committed, *before* any `tasks_claim_next` / starting the next task.
+- **Behaviour when present:** print the SESSION BOUNDARY line verbatim, then stop (do not
+  claim the next task).
+- **Behaviour when absent:** unchanged — claim / start the next task as usual.
+
+### Verbatim message
+
+```
+🔚 SESSION BOUNDARY — [slug] закрыта. Рекомендую завершить текущую сессию. Следующий трек: [value | "см. STATUS.md"]
+```
+
+`[slug]` = the closed task's slug. `[value | "см. STATUS.md"]` = the marker's string value,
+or the literal `см. STATUS.md` when the marker is just `true`. The wording is fixed so the
+boundary is greppable and recognisable across sessions.
+
+## Why a marker, not a heuristic
+
+The decision of *what counts as a stopping point* belongs to whoever scoped the work (the
+delegating workshop), not to the runner mid-flight. A heuristic ("stop after N tasks", "stop
+when tired") would either over- or under-fire. An explicit, opt-in marker keeps the default
+unchanged and makes the boundary a deliberate authoring choice.
+
+## Versioning
+
+MINOR bump (1.1.0 → 1.2.0): new optional capability, no existing behaviour changed.