Extracts the #45 write/update/get logic + the wiki upkeep that must
accompany a write (wing _index rebuild, cross-wing auto-tunnel, graph
re-index) into a single concrete brainstore.Store implementing
capture.BrainStore. The MCP brain_write/brain_update/brain_get handlers
are re-pointed at it, so there is one implementation, not two — the DRY
payoff #51 is named for. capture and MCP now share the exact same brain
write path and read-after-write contract.
The Server gains a *brainstore.Store, constructed in NewServer and given
the graph store in WithGraph. Embedding refresh stays out-of-band
(mtime-driven vectorstore.Sync), unchanged. Existing MCP brain_update/
brain_get/brain_write tests pass unmodified — behaviour and the
{id, path, content_hash} response contract are preserved.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>