feat(capturehttp): POST /capture REST adapter + OAuth2 + origin resolver (#53)

The HTTP door for the capture capability. Thin: authenticate → derive
trust-zone origin → decode → capture.Service → map receipt to status.

- Auth mirrors the chassis Bearer precedence (static token wins, then Dex
  JWT) but returns the resolved principal + auth path, which the chassis
  middleware hides — capture needs the principal to derive the origin.
  Depends on a small Validator interface (the chassis *JWTValidator
  satisfies it) so the JWT/origin path is testable without a live JWKS.
- OriginResolver maps principal → trust zone: static-token caller and
  allowlisted JWT subjects → sovereign; every other principal → us-nexus
  (fail safe, so the I1 gate refuses confidential by default). Principal
  and origin are server-set on the input, overwriting any body the caller
  sent.
- HTTP status: 200 all-ok / dry-run, 207 partial, 502 all-failed, 403 on
  the I1 refusal, 400 on fail-closed validation.
- Wired in main behind the same static+JWT credentials as /mcp, reusing
  the MCP server's graph-wired brain store (one implementation) and the
  classification tags (#50). Mounts only when a Gitea tracker is
  configured. Sovereign JWT principals via BRAIN_CAPTURE_SOVEREIGN_PRINCIPALS.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/internal/mcp/server.go

@@ -116,6 +116,13 @@ func (s *Server) IssueTracker() capture.IssueTracker {
 	return s.tracker
 }
 
+// BrainStore returns the shared brain store (graph-wired once WithGraph
+// has run), so the capture use-case writes through the exact same
+// implementation as the MCP handlers.
+func (s *Server) BrainStore() *brainstore.Store {
+	return s.store
+}
+
 func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	// MCP streamable HTTP: GET establishes the SSE stream for server-to-client events.
 	if r.Method == http.MethodGet {