feat(rag): add pure Python chunking and RRF foundation

[adihex]

Summary

• add a corpus-free, standard-library Python core under apps/rag-pipeline
• provide deterministic sentence-aware overlapping text chunking
• provide generic weighted reciprocal-rank fusion with stable tie handling
• cover boundaries, validation, weighting, duplicate handling, and ordering with focused unit tests

This is an independent foundation slice replacing the pure reusable portion of #10. It has no dependency on #10 or any other split/slice. It intentionally excludes Chroma/Gemini clients, FastAPI, NotebookLM integration, downloaded sources/data, notebooks, generated wiki content, duplicated agent configuration, and root hook/tooling changes. This PR does not modify or close #10.

Verification

• python3 -m unittest discover -s tests -v (from apps/rag-pipeline): 10 passed
• mise exec -- pnpm --filter @agentx/rag-pipeline test: 10 passed
• python3 -m pip wheel --no-deps --wheel-dir <temp> .: built agentx_rag_core-0.1.0-py3-none-any.whl
• mise exec -- pnpm build: passed (19 workspace projects)
• git diff --check: passed

Existing root-check failures

• mise exec -- pnpm test: 360 passed, 1 skipped, 1 failed. The pre-existing packages/core/tests/injection.test.ts custom ADP handler assertion also fails when run alone; this PR does not touch that code.
• mise exec -- pnpm check: pre-existing formatting findings in 23 files outside this PR. The new package.json passes the formatter; the scoped lint phase reports no lintable files.

The initial red TDD run failed with ModuleNotFoundError: No module named 'rag' before implementation; the final Python suite is green.

[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.