Agents: real ClawBrainHub integration (list + pull brains → trading cards)

Backend:
- brainhub.rs: BrainHubClient trait + HttpBrainHubClient (public discovery API,
  optional Bearer token) + FakeBrainHubClient. Lists the user's brains
  (CLAWBRAINHUB_OWNER, default "omar") + redclawsystems reference brains, and
  pulls a .brain JSON, parsing identity/skills/tools into card fields
  (tagline, skills, tools, capabilities). 2 parser unit tests.
- Routes GET /v1/brainhub/brains + /v1/brainhub/pull (rate-limited); wired into
  AppState + main (HttpBrainHubClient) + test harness (FakeBrainHubClient).

Mobile:
- src/api/brainhub.ts (listBrains/pullBrain), draftStore.pendingAgentBrain.
- app/pick-brain.tsx: pick a brain → pull → hand to the agent-card editor.
- Agents "+ New agent" now opens the brain picker; demo.tsx adopts the pulled
  brain (name, tagline, skill bars, tools, capabilities) for the flip side.

Verified live against clawbrainhub.com (omar's brains + reference brains list;
general-assistant pull yields real skills). Token stays server-side/env only.
Gate: mobile 28, backend 123.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

cardclaws-backend/crates/cardclaws-api/tests/common/mod.rs

@@ -20,6 +20,7 @@ use tower::ServiceExt;
 
 use cardclaws_api::ai::FakeAiClient;
 use cardclaws_api::assets::InMemoryStore;
+use cardclaws_api::brainhub::FakeBrainHubClient;
 use cardclaws_api::cache::InMemoryCache;
 use cardclaws_api::email::CapturingEmailSender;
 use cardclaws_api::geo::{GeoLocation, GeoResolver};
@@ -86,6 +87,7 @@ pub async fn try_setup() -> Option<TestApp> {
         assets: assets.clone(),
         geo: Arc::new(FakeGeo),
         ai: Arc::new(FakeAiClient),
+        brainhub: Arc::new(FakeBrainHubClient),
         jwt: JwtKeys::new("test-jwt-secret"),
         apple: Arc::new(NoopApple),
         apple_audience: "com.cardclaws.test".into(),