feat(mcp): hybrid-ranked search_code + get_neighbors + get_file_neighbors

Nav baseline rebased onto the #681 (MCP tools) + #698 (analyzer IMPORTS/OVERRIDES
edges) confluence, dropping the duplicate analyzer-edges copy (722c8a3, byte-identical
to #698's ce4ecd9) and the T13/T14 onboarding-template edits (deferred).

Adds get_neighbors (symbol-id neighbor expansion over CALLS/IMPORTS/EXTENDS/OVERRIDES),
get_file_neighbors (file-level structural coupling), get_importers/get_overrides spikes,
and a hybrid (BM25 + structural) ranking for search_code. Cherry-pick of 799b218.

PR-split note: get_file_neighbors + _resolve_file + FILE_NEIGHBOR_RELS form a
self-contained trailing block (PR N2); everything else is PR N1.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: DvirDukhan