chore(brain): remove brain_write legacy params domain/type/knowledge-path (ADR-0012)

hyperguild#79's caller audit (done in a prior session) found the removal
was safe:

- The promote/pending flow (#38/#81, ingestion/internal/api/promote.go)
  operates exclusively on brainDir/raw/ -> brainDir/wiki/<wing>/<hall>/. It
  never touches brainDir/knowledge/, so removing the knowledge/ write path
  does not break it.
- No live caller uses domain/type or the knowledge/ path: checked
  hyperguild itself, mathias/skills, mathias/agentsquad, mathias/dispatch,
  mathias/cad, mathias/cad-atlas.
- internal/skills/brain (a skill.go + handlers.go pair) defined a third,
  even-more-legacy brain_write shape (type/domain/filename, no wing/hall)
  for an old "supervisor" MCP server whose cmd/supervisor/main.go no
  longer exists. Nothing outside its own test imported it; deleted
  entirely as it encodes the exact legacy shape this issue removes.

Changes:
- ingestion/internal/mcp/handlers.go: brain_write's tool schema and
  brainWriteArgs drop type/domain; wing+hall are now marked required in
  the schema.
- ingestion/internal/api/handler.go: WriteNote has one behaviour — wing+
  hall required, routing to wiki/<wing>/<hall>/. writeLegacyNote and the
  knowledge/-path branch are deleted. Omitting wing or hall now returns
  "wing and hall are required" (400 over HTTP, a tool error over MCP) —
  the only sane failure mode left once there is no other valid
  destination.
- ingestion/internal/capture/ports.go, internal/brainstore/store.go: drop
  Type/Domain from capture.Note and its WriteNoteOptions construction.
  pipeline/*.go's unrelated "domain" field (a different, legitimate
  concept for brain_ingest_raw) is untouched.
- internal/skills/brain/: deleted (orphaned legacy tool schema, dead
  since cmd/supervisor was removed).

Tests: added TestWriteNote_RequiresWingAndHall (api),
TestBrainWriteRequiresWingAndHall +
TestBrainWriteIgnoresLegacyTypeAndDomainParams (mcp) for the new single
behaviour; removed tests that only exercised the deleted legacy path
(TestWrite_IncludesFrontmatterWhenTypeProvided,
TestWriteNote_HallRouteExistingTypeWinsOverOptsType,
TestBrainWriteRejectsTraversal, TestBrainWriteAcceptsDoubleDotInName —
the latter two's filename-sanitisation behaviour is covered independently
by internal/brain/path_test.go's TestNotePath_SanitisesWingAndSlug /
TestSanitise, which is unaffected by this change).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ingestion/internal/brainstore/store.go

@@ -51,8 +51,6 @@ func (s *Store) Write(ctx context.Context, n capture.Note) (capture.Ref, error)
 	relPath, err := api.WriteNote(s.brainDir, api.WriteNoteOptions{
 		Content:  n.Content,
 		Filename: n.Filename,
-		Type:     n.Type,
-		Domain:   n.Domain,
 		Wing:     n.Wing,
 		Hall:     n.Hall,
 	})