feat(miner): migrate policy-doc-cache onto the SqliteDriver store seam

[JSONbored]

Summary

• Continues Migrate AMS's local-store layer (node:sqlite) to the shared pg-adapter/SqliteDriver seam #7175/feat(miner): SqliteDriver store seam + migrate run-state #7194's storage-abstraction pattern -- currently only run-state.js had been
  migrated onto the SqliteDriver seam (store-db-adapter.js), despite Migrate AMS's local-store layer (node:sqlite) to the shared pg-adapter/SqliteDriver seam #7175 closing with the
  broader migration still ~1/13 done (see feat(miner): finish rolling the SqliteDriver seam onto AMS's remaining local stores #7282, updated to track the accurate remaining scope).
• policy-doc-cache.js's get/put now route through openLocalStoreAdapter's driver.query
  instead of raw DatabaseSync-backed prepared statements, matching run-state.js's exact
  pattern.
• Public API stays fully synchronous -- this is a pure seam migration (routing through an
  abstraction that could later back onto a different driver), not a behavior or logic change.
  Proven by every existing test passing completely unmodified.

Advances #7282

Scope

• The PR title follows type(scope): short summary Conventional Commit format.
• This PR is focused (one store migrated, no unrelated changes) and does not mix unrelated
  backend, UI, MCP, docs, dependency, and deploy changes.
• This follows CONTRIBUTING.md and does not reintroduce GitHub Pages, VitePress, site/, or
  CNAME.
• I linked a currently open issue this PR advances (Advances #7282 above -- deliberately not
  Closes, since feat(miner): finish rolling the SqliteDriver seam onto AMS's remaining local stores #7282 tracks migrating the remaining ~11 stores too, not just this one).

Validation

• git diff --check
• npm run typecheck (root -- clean)
• npm run test:coverage on the changed module -- 100% statements/branches/functions/lines
  (28/28, 14/14, 7/7, 24/24), verified against the raw coverage report.
• Every existing test in test/unit/miner-policy-doc-cache.test.ts and
  test/unit/opportunity-fanout-policy-doc-cache.test.ts (20 tests total) passes completely
  unmodified -- direct evidence this migration changed no observable behavior.
• Full npm run test:ci green end to end, run under the repo's pinned Node 22 (.nvmrc), on a
  clean coverage directory (a prior run hit a transient coverage-tmp-file race from overlapping
  local gate invocations; re-run clean to confirm).
• npm audit --audit-level=moderate -- clean except the same pre-existing, unrelated
  adm-zip/github-actionlint advisory with no fix available.

Safety

• No secrets, wallet details, hotkeys, coldkeys, user PATs, private keys, raw trust scores,
  private rankings, or private maintainer evidence are exposed.
• Public GitHub text stays sanitized, low-noise, and does not imply compensation guarantees or
  optimization tactics.
• Auth, cookie, CORS, GitHub App, Cloudflare, or session changes include negative-path tests.
  (N/A -- local SQLite cache store, not an auth surface.)
• API/OpenAPI/MCP behavior is updated and tested where needed. (N/A -- no OpenAPI-documented
  surface touched.)
• UI changes use live API data -- N/A, no UI change.
• Visible UI changes include a UI Evidence section -- N/A, backend-only change, no visible UI.
• Public docs/changelogs are updated where needed; changelogs are only edited for
  release-prep PRs. (No CHANGELOG.md edit; internal storage-abstraction detail.)

Notes

• policy-doc-cache.js was chosen as the next store to migrate for being the smallest (79 lines,
  no interactive transactions, no RETURNING statements to trip store-db-adapter.js's own
  documented read/write heuristic limitation).
• feat(miner): finish rolling the SqliteDriver seam onto AMS's remaining local stores #7282 has been corrected to describe the real remaining scope of this migration (the seam exists
  and is proven; the actual Postgres driver and interactive-transaction support are separate,
  larger follow-on efforts, not part of this PR or feat(miner): finish rolling the SqliteDriver seam onto AMS's remaining local stores #7282's current scope).

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.