docs: plan concurrent task execution; archive completed tasks plan

New plan grounds three concurrency issues found by tracing the actual code
(not assumed): the assent/destructive windows are keyed by agent id only
(no session dimension), so an approved plan in one task can auto-run
unapproved actions in a concurrently-running task; nomos shares one
mutex-guarded MCP client across all sessions, so a single slow `run` call
serializes every other task's tool calls behind it; and chat.ts's SSE
callback has no session guard, so switching tasks mid-stream lets the
backgrounded task's events corrupt whatever's now displayed. Proposes
session-scoping the windows (critical/first), a frontend stream guard
(contained/second), a per-session MCP client pool (throughput/third), and
an optional concurrency cap (deferred pending real usage data).

Also archives the goal-oriented-chat-control-panel plan to done/ — all 7
phases shipped and are live in production (SHA e30813a) — fixing its
internal relative links for the new depth and pointing forward to the new
concurrency plan as follow-up hardening.

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

plans/index.md

@@ -13,7 +13,7 @@ went sideways, open an investigation.
 | 2026-07-08 | [Control room web UI](2026-07-08-control-room-webui.md) | In Progress |
 | 2026-07-08 | [Liveness, drift, and UX cohesion](2026-07-08-liveness-drift-and-ux-cohesion.md) | In Progress — Phase 5 deferred |
 | 2026-07-10 | [General gated execution: unlimited actions, gated by risk](2026-07-10-general-gated-execution.md) | In Progress — enum retirement + auto-act revival still open |
-| 2026-07-11 | [Tasks: the chat page as goal-structured autonomous work](2026-07-11-goal-oriented-chat-control-panel.md) | Planned |
+| 2026-07-11 | [Concurrent task execution: safety + throughput + frontend correctness](2026-07-11-concurrent-task-execution.md) | Planned |
 
 ## Done
 
@@ -39,6 +39,7 @@ See [`done/`](done/) for executed plans:
 | 2026-07-09 | [Chat sessions: reliability, cost, and session-management fixes](done/2026-07-09-chat-sessions-improvements.md) |
 | 2026-07-09 | [Session execution, UX, and learning improvements](done/2026-07-09-session-execution-and-ux-fixes.md) |
 | 2026-07-10 | [Autonomous plan execution: close the observation gap](done/2026-07-10-autonomous-plan-execution.md) |
+| 2026-07-11 | [Tasks: the chat page as goal-structured autonomous work](done/2026-07-11-goal-oriented-chat-control-panel.md) |
 
 ## Conventions