fix(merge): restore chat.js imports, trust-vision floor, gitignore entries

Audit of every branch-owned line the -X theirs merge dropped from
the 32 pre-merge commits found three more real regressions:

* src/sse/handlers/chat.js: merge kept the capsOverride feature
  (bb8d67ba) but reverted the import block, so getCustomModels and
  capabilitiesFromServiceKind were undefined. The runtime error was
  swallowed by the feature's own fail-open try/catch — custom
  models silently lost their vision override. Restored both imports.
* open-sse/providers/capabilities.js: TRUST_UPSTREAM_VISION (the
  floor that keeps vision on for unknown models on upstream-validating
  gateways like openrouter) was left as dead code by the merge —
  upstream rewrote step 4 as refine() and dropped the check.
  Re-applied it on top of the new refine() so catalog/limits
  refinement still applies.
* tests/unit/chat-connection-pin.test.js: mock auth module lacked
  isModelAllowedForKey added by f0adfb20.
* .gitignore: re-add .pi-subagents/.

tests/unit/chat-connection-pin.test.js

@@ -16,6 +16,7 @@ const authMocks = vi.hoisted(() => ({
   clearAccountError: vi.fn(async () => {}),
   extractApiKey: vi.fn(() => null),
   isValidApiKey: vi.fn(async () => true),
+  isModelAllowedForKey: vi.fn(() => true),
 }));
 const tokenMocks = vi.hoisted(() => ({
   checkAndRefreshToken: vi.fn(async (_p, creds) => creds),