chore(refactor): legacy post-Wave-1 cleanup (#45, #46, #47, #51)

[4nibhal]

Summary

Remove dead code and consolidate types left behind by Wave 1
(bhodi -> bodhi-rag rename).

• Drop the dead langchain shim in src/bodhi_rag/indexing/
  (~162 lines, never executed in production).
• Fix the __all__ lie in src/bodhi_rag/interfaces/__init__.py
  (advertised tui and worker submodules that did not exist).
• Consolidate RetrievedDocument on the domain dataclass; delete
  the duplicate Protocol in ports/vector_store.py.
• Drop the dead _merge_document_metadata helper and orphaned
  RESERVED_PROVENANCE_KEYS constant from application/facade.py.

Architecture

No behavioral changes. Pure code removal + type consolidation.

• The langchain shim was residual post-migration code: the
  production code path goes through the cross-context ports
  (DocumentParserPort / ChunkerPort / EmbeddingPort /
  VectorStorePort) wired by the composition root.
• The __all__ lie advertised non-existent submodules; a
  contributor trusting __all__ and writing
  from bodhi_rag.interfaces import tui would have hit an
  immediate ImportError.
• The RetrievedDocument Protocol was a duck-typed shadow of
  the real domain dataclass. Ports now reference the dataclass
  directly so strict type checkers (mypy --strict, pyright
  --strict) resolve port signatures against the real shape.
• The dead _merge_document_metadata helper was a refactor
  regression from commit 864c573; the canonical implementation
  lives in indexing/application/index.py as
  _build_merged_document.

Testing

Each commit was validated locally with uv:

• uv run ruff check src tests -> all checks passed
• uv run mypy src/ -> no issues found in 90 source files
• uv run pytest -> 259 passed (2 tests removed were the
  characterization tests of the deleted dead code)

Total diff: 12 files changed, 12 insertions(+), 303 deletions(-).

Closes #45, #46, #47, #51