feat(search): index prose content for BM25 full-text search

[ShauryaaSharma]

What & why

search_graph BM25 only matched node names and headings, so it was blind to the
prose that documentation- and config-heavy repos carry. Markdown Section nodes
exposed only their heading; YAML/JSON Module nodes only their file name — the
section body and the description value were never indexed, and Section/Module
were excluded from BM25 results entirely. This indexes that prose so content is
searchable.

Closes #518
Closes #519

Changes

• store (store.c/.h): add a body column to the nodes_fts FTS5 table; new
  cbm_store_fts_rebuild() drops+recreates it (upgrading legacy 4-column DBs) and
  backfills body from each node's docstring, guarded by json_valid().
• pipeline: both FTS backfill sites call the shared cbm_store_fts_rebuild().
• mcp: stop excluding Section/Module from BM25 results (no rank boost, so code
  results still sort first).
• internal/cbm: capture the markdown section body (Section nodes don't index body text — BM25 can't search markdown content #518) and promote a top-level
  description/summary/purpose value onto the Module node (META.yaml/frontmatter description values not indexed for BM25 search #519), reusing the
  existing docstring property.

Testing

7 extraction cases + 3 store FTS cases added. Verified end-to-end: bodies are
extracted → indexed into nodes_fts.body → returned by BM25; json_valid() tolerates
malformed rows; legacy FTS tables upgrade on rebuild.

Notes

Backward compatible (additive column; legacy DBs upgrade on next index). No MCP
tool changes, no new deps, no new system()/popen()/network calls. #518 and #519
share the FTS body infra (#519 can't work without it), so they're together —
happy to split if preferred.

[ShauryaaSharma]

Rebased onto main (post-#667) , all tests green ✅

Rebased on top of the #667 merge. One minor conflict in extract_defs.c: upstream added qn_safe_segment() immediately before push_simple_class_def as part of the markdown QN slugification work. Resolution was additive, kept both the new helper and the docstring parameter this PR adds.

Verification against the #518 repro in the bug suite:

The #667 markup battery (repro_grammar_markup_markdown) now correctly expects "Section" as the label for headings, which is exactly what this PR emits. Running the 7 targeted extraction tests locally confirms the rebased code is still correct:

markdown_section_body_captured          PASS  ← Playwright body captured, sibling excluded
markdown_section_no_body                PASS  ← empty heading → no docstring
markdown_section_body_capped            PASS  ← body ≤ 500 bytes
yaml_description_promoted_to_module     PASS
yaml_summary_promoted_to_module         PASS
json_description_promoted_to_module     PASS
yaml_no_description_leaves_module_bare  PASS

[DeusData]

Huge thanks for opening this PR and for the work you put into it.

The maintainer shop is currently full, so this may sit for a bit before it gets a proper review. We will come back to this as soon as possible with real feedback; I wanted to make sure it did not sit unacknowledged in the meantime.

[ShauryaaSharma]

No worries at all @DeusData, take your time, there's no rush on my end! Happy to wait and appreciate you acknowledging it. 🙂