docs(ai-chat): задокументировать наблюдаемую поверхность #487 (F2)

- CHANGELOG [Unreleased]: серверный supersede + три кода (SUPERSEDE_INVALID /
  SUPERSEDE_TARGET_MISMATCH / SUPERSEDE_TIMEOUT) в Added; смена поведения
  (легаси вторая вкладка теперь → 409 A_RUN_ALREADY_ACTIVE вместо второго
  параллельного стрима; каждый ход — ран в обоих режимах) в Changed.
- .env.example: три новых AI_CHAT_* (SUPERSEDE_TIMEOUT_MS / RECONCILE_INTERVAL_MS
  / INAPP_TOOL_CALL_CAP_MS, дефолты 10000/120000/120000, связь cap↔staleness).
- AGENTS.md:458: ран теперь универсален (оба режима), флаг autonomousRuns =
  только disconnect-семантика; плюс упоминание supersede.
- ai-chat.service.ts:1136: устаревший коммент про «legacy → socket signal» →
  оба режима на run-signal, guard runId защищает лишь no-handle fallback.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.env.example

@@ -288,6 +288,29 @@ MCP_DOCMOST_PASSWORD=
 # registry is process-local).
 # AI_CHAT_RESUMABLE_STREAM=false
 
+# --- Run lifecycle tunables (#487) ---
+# These govern the universal run machinery (every turn is now a first-class run,
+# both modes) and rarely need changing.
+#
+# How long a server-side SUPERSEDE ("interrupt and send now") waits for the target
+# run to settle after issuing Stop before it degrades to a 409 SUPERSEDE_TIMEOUT
+# (nothing sent, the composer keeps the user's text). 10s is generous under a
+# healthy DB; do NOT raise it to paper over a slow DB — a SUPERSEDE_TIMEOUT is the
+# honest signal. Default 10000 (10s).
+# AI_CHAT_SUPERSEDE_TIMEOUT_MS=10000
+#
+# How often the periodic bidirectional reconcile job runs (heals runs/messages
+# left dangling by a crash or a lost terminal write). Default 120000 (2 min).
+# AI_CHAT_RECONCILE_INTERVAL_MS=120000
+#
+# Wall-clock cap for a SINGLE in-app tool call (a long paginated read, or a content
+# write whose collab commit hangs) — the per-call half of the composite abort
+# signal every in-app tool is wrapped with (the other half is the turn's Stop).
+# The reconcile staleness floor is derived as max(2 x this cap, 15min), so a very
+# high value delays stale-run recovery (the server boot-warns above 30min). Default
+# 120000 (2 min).
+# AI_CHAT_INAPP_TOOL_CALL_CAP_MS=120000
+
 # --- Anonymous public-share AI assistant ---
 # Opt-in per workspace (AI settings -> "public share assistant"; off by default).
 # When enabled, anonymous visitors of a published share can ask an AI about that