fix(plans): /revise route now dispatches plan_update prompt

[pufit]

Summary

The web UI's Revise button silently no-op'd. The HTTP route /api/plans/{plan_id}/revise still instructed the planner to call plan_propose, which now refuses with "Task X already has a pending or implementing plan. Skip it." whenever a pending plan exists for the task — i.e. every revise attempt. The request returned 200 {\"status\": \"revision_requested\"}, so the failure was invisible.

Fix: dispatch the plan_update prompt (mirroring the MCP plan_revise tool) and extract a shared request_plan_revision() helper so the two surfaces can't drift apart again.

Changes

• New nerve/agent/plan_service.py — single source of truth for revision dispatch. Exposes request_plan_revision(db, engine, plan_id, feedback) plus PlanNotFound / TaskNotFound / PlanNotPending exceptions. Prompt template is a module-level constant.
• HTTP route nerve/gateway/routes/plans.py — thin wrapper around the helper; maps exceptions to 400 (empty feedback), 404 (plan/task missing), 409 (non-pending). Previously every error path returned 200 with feedback silently stored on the plan.
• MCP tool nerve/agent/tools.py::plan_revise — same thin-wrapper treatment.
• Frontend — planStore.revisePlan now returns Promise<boolean>, sets actionError on failure, and parses FastAPI's {detail: ...} JSON envelope. PlanDetailPage renders the error inline next to the textarea; the form only collapses on success.

Test plan

• tests/test_plan_revise.py — 17 new tests covering the helper (prompt content, feedback persistence, session routing, fallback to cron:task-planner, all three exceptions, empty-feedback ValueError), the MCP tool wrapper, and the HTTP route via FastAPI TestClient.
• Full suite green: 564 passed in 33s
• npm run build clean
• Manual smoke test post-merge against one of the 85 pending plans:

  curl -X POST http://localhost:8900/api/plans/<some-pending-plan>/revise \
       -H \"Content-Type: application/json\" \
       -d '{\"feedback\":\"smoke test — be more specific about edge cases\"}'
  # Expect: original plan superseded + v2 pending within ~60s

Out of scope

The dead supersede-loop in _plan_propose_impl (tools.py:1131-1133) is now unreachable from the revise path. Harmless dead code; separate cleanup PR.

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