feat(mcp): rewrite search_codebase description to lead with unique value

Old description told the agent to 'Use this INSTEAD of grep' — models
with strong native Grep affordances (Claude Code, Cursor) routinely
ignore that instruction because grep feels cheap and they've used it
ten thousand times.

New description leads with what's *only* possible here: related-files
(co-change history) and definitions enrichment grep can't give. Concedes
literal-string lookups to grep explicitly, which makes the tool's actual
edge — structural connectedness — more legible to the model.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>