Adds the `brain_tunnel` MCP tool and auto-tunnel behaviour for
`brain_write`, so concepts that appear in multiple wings become
navigable from any of them.
New surface in package brain:
- WriteTunnel(brainDir, src, tgt) — appends a `## See also` bidirectional
wikilink between two notes in different wings. Idempotent (link not
duplicated on re-call) and reuses an existing See also section.
- DetectTunnels(brainDir, content) — walks brain/wiki/, returns
TunnelCandidates for notes whose title appears in content. Tags
whole-word case-insensitive hits as Exact=true and substring-only hits
as Exact=false.
- AutoTunnel(brainDir, src, content) — wraps DetectTunnels: writes
cross-wing exact matches, stages fuzzy matches into
brain/raw/tunnel-candidates-<YYYY-MM-DD>.md for human review.
MCP wiring:
- `brain_tunnel` tool: explicit manual link (source, target).
- `brain_write` with wing+hall now triggers AutoTunnel on the new
content. Failures are logged and never abort the primary write.
readTitleAndCreated also humanises the slug fallback (hyphens → spaces)
so titleless notes participate in content matching.
Closes hyperguild#16.
Tests: idempotency, same-wing rejection, missing-note rejection,
See-also reuse, exact/fuzzy detection, slug fallback, MCP tool happy
path, auto-tunnel hook (cross-wing exact → linked; same-wing → skipped;
fuzzy → candidates file).
