feat(rag): add provider-independent pipeline runtime

[adihex]

Summary

• define typed injected dense/lexical retriever and generator protocols
• fuse retrieval rankings with the existing reciprocal rank fusion core
• build deterministic labeled context and preserve source citation metadata
• return an explicit insufficient-information result without invoking generation

Scope

Corpus-free and provider-independent. No Gemini, Chroma, FastAPI, notebook, downloaded corpus, path, or network-test dependencies.

Tests

• mise exec -- pnpm --filter @agentx/rag-pipeline test
• python3 -m pip wheel --no-deps --wheel-dir /tmp/agentx-rag-wheel . (from apps/rag-pipeline)
• mise exec -- pnpm build

Stacked on #17 / split/rag-core. Replacement slice for conflicted #10; do not merge automatically.

[adihex]

Landed on main as part of the consolidated, rebased stack ending at e9bd4bd. Verified this PR has no patches absent from origin/main, so closing the now-redundant stacked PR.