feat(cursor): auto-migrate sessions to healthy account on quota exhaustion

When a Cursor account's quota is exhausted, sessions bound to it can now
seamlessly continue on a different account:

Layer 1 — Checkpoint decoupling:
  Key checkpoints by conversationId (not authID:conversationId). Store
  authID inside savedCheckpoint. On lookup, if auth changed, discard the
  stale checkpoint and flatten conversation history into userText.

Layer 2 — Cross-account session cleanup:
  When a request arrives for a conversation whose session belongs to a
  different (now-exhausted) auth, close the old H2 stream and remove
  the stale session to free resources.

Layer 3 — H2Stream.Err() exposure:
  New Err() method on H2Stream so callers can inspect RST_STREAM,
  GOAWAY, or other stream-level errors after closure.

Layer 4 — processH2SessionFrames error propagation:
  Returns error instead of bare return. Connect EndStream errors (quota,
  rate limit) are now propagated instead of being logged and swallowed.

Layer 5 — Pre-response transparent retry:
  If the stream fails before any data is sent to the client, return an
  error to the conductor so it retries with a different auth — fully
  transparent to the client.

Layer 6 — Post-response error logging:
  If the stream fails after data was already sent, log a warning. The
  conductor's existing cooldown mechanism ensures the next request routes
  to a healthy account.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/auth/cursor/proto/h2stream.go

@@ -205,6 +205,10 @@ func (s *H2Stream) Data() <-chan []byte { return s.dataCh }
 // Done returns a channel closed when the stream ends.
 func (s *H2Stream) Done() <-chan struct{} { return s.doneCh }
 
+// Err returns the error (if any) that caused the stream to close.
+// Returns nil for a clean shutdown (EOF / StreamEnded).
+func (s *H2Stream) Err() error { return s.err }
+
 // Close tears down the connection.
 func (s *H2Stream) Close() {
 	s.conn.Close()