fix: reset shared index state on force rebuild

[Helweg]

Summary

Make force=true behave like a true shared index reset so provider/model dimension changes cannot reuse stale SQLite embeddings during the next rebuild.

Changes

• add a native database reset path that clears shared embeddings, chunks, branch catalogs, symbols, and call edges
• update Indexer.clearIndex() to wipe persisted shared index state and failed-batch state before rebuilding
• add regression coverage for an in-process embedding dimension switch during a force rebuild

Testing

How were these changes tested?

• Unit tests added/updated
• Manual testing performed
• Build passes (npm run build)
• Typecheck passes (npm run typecheck)
• Tests pass (npm run test:run)
• Lint passes (npm run lint)

Release Labels

• Added at least one release category label (feature, bug, performance, documentation, dependencies, refactor, test, chore, or skip-changelog)
• Added at most one semver label (semver:major, semver:minor, semver:patch) when needed

Related Issues

Fixes #61