feat(brain): stamp server-owned write provenance; surface it in brain_query

brain_write was an ungated, durable prompt-injection path: a session that
just read an untrusted page could write an entry every later session
retrieves as trusted guidance (OWASP MCP06). Decision (#145): tag
provenance rather than stage writes.

Every write and update stamps origin (static | jwt | unknown) and
written_by (the authenticated principal) into frontmatter. Server-owned:
origin/written_by in caller content or an update fragment are discarded,
so a caller cannot claim a more trusted origin. No principal stamps
origin: unknown, never silence. brain_update re-stamps to the last
writer. capture (the claude.ai connector door) stamps too, via a new
server-derived CaptureContext.AuthPath set by both capture doors after
decode. REST POST /write carries no principal and stamps unknown.

brain_query results carry origin/written_by when the note has them;
older notes omit them rather than fabricate. brain_get already returns
frontmatter.

Refs #145

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/capture/ports.go

@@ -40,6 +40,11 @@ type Note struct {
 	Wing     string
 	Hall     string
 	Reason   string
+	// Provenance (hyperguild#145) is stamped by the server from the
+	// authenticated principal, never taken from Content's frontmatter.
+	// Origin is "static", "jwt" or "unknown"; empty means "unknown".
+	Origin    string
+	WrittenBy string
 }
 
 // BrainStore is the brain persistence port — the shared implementation of