fix(#4141): ignore Stryker's sandbox in eslint's global ignores (#4178)

* fix(#4141): ignore Stryker's sandbox in eslint's global ignores

`stryker.config.mjs` sets `tempDirName: '.stryker-tmp'`, `.gitignore` ignores
it, and Stryker's own always-ignored list carries it. eslint's global
`ignores` did not, and there is no `.eslintignore`.

A mutation run that dies before cleanup — chunk timeout, CI cancellation,
Ctrl+C, a crash — leaves a full sandbox copy of the tree at
`.stryker-tmp/sandbox-*/`, and `npm run lint` then lints the copy. The
failure is not the duplicate-file failure that would suggest, which is what
makes it confusing: the `local` plugin is registered only in path-scoped
config blocks (`tests/**/*.cjs`, `scripts/**/*.cjs`, `hooks/**`), and a copy
at `.stryker-tmp/sandbox-*/tests/*.cjs` matches none of them, so every inline
`// eslint-disable-next-line local/<rule>` the original carries becomes
`Definition for rule 'local/<rule>' was not found` at the copy's path. The
error text names rules rather than paths, so the first read is "my change
broke the local plugin" — observed as 834 errors on a branch whose own diff
was clean, from a sandbox eight days stale.

CI never sees this (fresh checkout), so it is purely a local-contributor tax,
but a loud and misleading one.

The regression test asserts the invariant the way ESLint resolves it —
`isPathIgnored()` over real flat-config precedence, not a textual scan of the
ignores array — mirroring the #551 block it sits beside, and carries the
control that the real `tests/worktree.test.cjs` at the mirrored path is still
linted, so a pattern widened enough to swallow the tree cannot pass.

Scoped to the reported bug: `reports/` may deserve the same treatment, but no
lint failure has been reproduced from it, so it is not claimed here.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017zgWzB96uz3LVKdJePYTLR

* chore(#4141): backfill the changeset PR number

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017zgWzB96uz3LVKdJePYTLR

* chore(#4141): restore the file's 2-blank-line block separator

Review nit: the #4141 block left three blank lines before the RETIRED
block, where every other block boundary in this file uses two.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mw7WhYnrWdY8cmj8NSJj4R

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: Tom Boucher <trekkie@nomorestars.com>

eslint.config.mjs

@@ -78,6 +78,11 @@ export default tseslint.config(
       '.worktrees/**',
       '.claude/**',
       'coverage/**',
+      // #4141: Stryker's sandbox (tempDirName in stryker.config.mjs, also gitignored
+      // and always-ignored by Stryker itself). A run that dies before cleanup leaves a
+      // copy of the tree here; linting it reports the path-scoped `local/*` rules as
+      // undefined, which reads as the plugin being broken rather than as scratch space.
+      '.stryker-tmp/**',
       '**/*.generated.cjs',
       // ADR-457: tsc-generated runtime artifact — lint the src/*.cts source, not the emitted .cjs.
       'gsd-core/bin/lib/claude-orchestration.cjs',