feat: add pluggable reranker and diversity eval gating

[Helweg]

Summary

Add a pluggable external reranker pipeline with intent-aware guardrails, diversity controls, and evaluation/CI coverage so reranking quality is measurable and protected against regression.

Changes

• add optional external reranker support with snippet-aware payloads, exact-definition bypass, intent-safe banding, and duplicate suppression in src/indexer/index.ts
• add reranker config, benchmark coverage, and integration/unit tests across ranking, search, and config behavior
• extend evaluation with deduped/raw top-k diversity metrics plus budget, baseline, and report support so CI can gate reranker-quality regressions

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

None.