FN-5893: require invariant-level bug regression coverage

Add invariant-first regression guidance to triage prompts and testing docs.

- add an AGENTS standing rule to cover bug-fix invariants across all known surfaces
- update standard, fast, and core triage prompt templates to require invariant-level regression tests
- add triage regression tests that lock the new wording into standard, fast, and core prompts
- link docs/testing guidance back to the new invariant-over-repro rule

Files changed:
 AGENTS.md                                    |  7 ++++++
 docs/testing.md                              |  1 +
 packages/core/src/agent-prompts.ts           |  3 ++-
 packages/engine/src/__tests__/triage.test.ts | 32 ++++++++++++++++++++++++++++
 packages/engine/src/triage.ts                |  6 ++++--
 5 files changed, 46 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-5893

Fusion-Task-Lineage: 31c082c6-2a0a-4619-a13b-94dad97eef62

packages/core/src/agent-prompts.ts

@@ -261,7 +261,7 @@ Follow this structure exactly:
 
 - [ ] {Specific, verifiable outcome}
 - [ ] {Specific, verifiable outcome}
-- [ ] Run targeted tests for changed files
+- [ ] Run targeted tests for changed files, asserting the invariant across all known surfaces (enumerate every provider/bridge, desktop + mobile breakpoints, and empty/undefined/populated data states)
 
 **Artifacts:**
 - \`path/to/file\` (new | modified)
@@ -330,6 +330,7 @@ files with assertions that run via a test runner. Typechecks and builds are NOT
 tests. Manual verification is NOT a test.
 
 - Each implementation step should include writing tests for the code being changed
+- For bug fixes, regression tests must assert the invariant across all known surfaces — enumerate every provider/bridge, desktop + mobile breakpoints, and empty/undefined/populated data states — not just the reported repro (see FN-5787/FN-5789/FN-5803 and FN-5751)
 - The final Testing step runs lint, the FULL test suite, and project typecheck when the repo exposes one
 - Specs must instruct executors to fix lint failures and quality-gate failures directly, even when the required edits extend beyond the original File Scope
 - If the project has no test framework, the Testing step must include setting one up