FN-5928: enforce surface enumeration for bug-fix invariants

Require bug-fix specs and reviews to enumerate affected surfaces and reject repro-only regression coverage.

- add a required `## Surface Enumeration` section to triage prompt templates and bug-fix planning guidance
- tighten reviewer guidance to block missing surface enumeration and repro-only regression tests
- document the canonical surface checklist in `docs/testing.md` and cover the new wording with prompt/reviewer tests

Files changed:
 AGENTS.md                                         |  6 ++--
 docs/testing.md                                   |  9 +++++
 packages/core/src/__tests__/agent-prompts.test.ts | 22 ++++++++++++
 packages/core/src/agent-prompts.ts                | 16 +++++++++
 packages/engine/src/__tests__/reviewer.test.ts    | 17 ++++++++++
 packages/engine/src/__tests__/triage.test.ts      | 41 ++++++++++++++++++++---
 packages/engine/src/reviewer.ts                   |  3 ++
 packages/engine/src/triage.ts                     | 24 +++++++++++++
 8 files changed, 131 insertions(+), 7 deletions(-)

Fusion-Task-Id: FN-5928
Fusion-Task-Lineage: 717ddcbe-f3a6-4589-ad90-4e640f7a9ff2

docs/testing.md

@@ -98,3 +98,12 @@ Prefer `it.each` over copy-pasted `it()` blocks. When trimming, keep: first case
 - Do **not** add net-new real-network calls, real-`setTimeout` polling loops, or mock-the-world component shells when a narrower seam exists.
 - Use the canonical taxonomy in **What NOT to write** and **What TO keep unconditionally** when deciding trim vs keep.
 - See `docs/test-speed-audit-FN-5048.md` for the measured baseline offender list and optimization priorities.
+
+### Surface Enumeration checklist
+
+Copy this checklist into a bug-fix task's `## Surface Enumeration` section and make the regression tests prove the invariant across every checked surface. See `AGENTS.md` → **Standing Rule: Fix the Invariant, Not the Repro (FN-5893)** for the enforced planning/review contract.
+
+- [ ] Providers / bridges / execution paths touched by the invariant
+- [ ] Desktop + mobile breakpoints / platforms that exercise the behavior
+- [ ] Empty / undefined / duplicate / populated data states
+- [ ] Shared hooks / components / modules / helpers reusing the logic