fix: reuse connected engine for frontmatter reindex

[adriankae]

Summary

• Pass the already-connected CLI engine into reindex-frontmatter instead of constructing a second, disconnected PGLite engine.
• Make reindexFrontmatterCli own/disconnect only engines it creates itself.
• Avoid false-positive effective_date_health warnings for empty date-like frontmatter fields.
• Add regression coverage for CLI wiring and doctor effective-date checks.

Why

In PGLite installs, gbrain reindex-frontmatter could fail with PGLite not connected. Call connect() first. because the CLI had already initialized an engine, but reindex-frontmatter created/used its own unconnected engine path. This also made downstream Hermes installs carry a local compatibility patch across upgrades.

doctor also treated the mere presence of event_date, date, or published frontmatter keys as evidence of parseable date metadata even when the values were empty, producing noisy effective-date warnings.

Test plan

bun test test/reindex-frontmatter.test.ts
bun test test/doctor.test.ts -t "effective_date_health ignores empty frontmatter date fields"

Both pass locally.

Note: running the full test/doctor.test.ts file in this container hits pre-existing 5s timeouts in unrelated takes_weight_grid tests while fresh PGLite migrations run; the new focused tests pass.

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