chore(#2665): add the changeset fragment this PR now needs

Caught by CI on the round-3 push, not by review: `changeset-lint` went from
OK_NO_USER_FACING_CHANGES to FAIL_MISSING_FRAGMENT.

Both prior rounds passed that gate because the diff was tests/ + scripts/ only,
and neither is a USER_FACING_PREFIX. Round 3 is the first commit in this PR to
touch src/ — the descriptor hoist in src/runtime-homes.cts — and src/ is on the
list. So the fragment requirement is a direct consequence of the remedy shape,
not something the earlier rounds missed.

src/runtime-homes.cts is the ONLY user-facing file in the whole PR diff
(23 changed files); everything else is tests/, scripts/, CONTEXT.md, the two
generated CONTEXT-INDEX.json artifacts, and examples/.

Typed `Added` rather than `Fixed` because that is what actually reaches a user:
new exported constants on a shipped module. The fix itself (#2665) is test
hermeticity, which changes no shipped behaviour — resolveKimiHooksTomlDir()
resolves identically before and after.

Worth noting the local lint disagreed with CI and was wrong: it diffs
`origin/main...HEAD`, which sweeps in the base's own committed fragments and
returns a false ok_fragment_present. CI diffs `origin/$GITHUB_BASE_REF...HEAD`
(next) and sees only this PR's files, which is the correct question.

.changeset/quiet-moons-derive.md

@@ -0,0 +1,5 @@
+---
+type: Added
+pr: 2677
+---
+**`runtime-homes` now exports its non-registry config-home descriptors** — `KIMI_HOOKS_TOML_DESCRIPTOR`, `NON_REGISTRY_CONFIG_HOME_DESCRIPTORS`, `GSD_LOCATION_ENV_KEYS`, and the `ConfigHomeDescriptor` type are public, so consumers that need the *set* of config-location env vars (rather than a single resolved path) can derive it instead of hand-maintaining a copy. `resolveKimiHooksTomlDir()` behaviour is unchanged; its descriptor is simply named rather than inline (#2665).