docs(changelog): GST-775 pregate lane policy fragment

Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>

changelog.d/capy-gst-775-pregate-lane-policy.md

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+- **A `.gstack/pregate.json` that only raises `timeout_s` no longer changes lane verdicts, `warn: ["lanes"]` no longer lets a failing test pass, and a `[]` declaration no longer skips a deleted test's suite** (GST-775, from gbrain-evals #272 and GBEV-22).
+  - Before, any policy file, even `{"timeout_s": 1800}`, made a touched file with no test lane a hard `PREGATE_NO_LANE` that stopped before any test ran. Now only a policy that declares lanes (a `dependencies`, `empty_lanes`, `selection` or `local` key, even an empty one) does that. A policy with none of them keeps the no-policy verdict: `warn` `PREGATE_NO_POLICY` with a starter, the selected tests run, and a red test is still `PREGATE_LANE_FAILED`. A hand-written policy that sets only `runner`, `guards`, `strays`, `allowlist` or `remote_gate` moves from the hard failure to that warning; add `dependencies` to keep unmapped files failing. See docs/pregate.md#lane-requirements.
+  - `warn: ["lanes"]` used to downgrade the missing-lane failure before the tests ran, so a red test beside an unmapped file passed. Now the selected tests run first. A red lane is `fail`, an unfinished one `incomplete`, a test only a platform lane runs stays `requires-remote`, and only a green run becomes `warn` `PREGATE_NO_LANE` naming the unmapped files.
+  - A `[]` declaration that matches a deleted test (the `"**": []` catch-all) used to clear it before the removed-test handling, so its owning suite did not run. Now the deleted test still runs its owning suite, and a file naming it that a `[]` covers, such as a durations list, counts as covered.