docs: remove CallCredentials — dead field, dead from_call path, auth_token is per-request payload

ADR-091 amended 2026-07-17: CallCredentials removed (not retained in
alknet-call). Trace showed CallCredentials.auth_token had no reader
(connect() read only tls_identity + remote_identity; spawn_dispatch
takes no credentials; from_call's credentials_auth_token was a
different type, always None, never connected). auth_token is a
per-request payload field — browsers send it in the WS payload; the
HTTP gateway resolves bearer → Identity at its boundary.

from_call's credentials_auth_token dead path removed in the same pass
(OpSummary field, handler params, build_forwarded_payload param, and
the two tests asserting the never-exercised Some path).

ADR-089 §5 further amended, ADR-080 noted, all spec READMEs and
overview updated. Migration plan (findings.md) corrected: Phase 5
prune now includes CallCredentials removal + from_call dead-path
removal; test audit corrected (4 unchanged + 2 move to core, not 6
unchanged); integration-test split documented; all 'or' hedges
resolved.

docs/architecture/questions/066-alknet-register-wire-protocol.md

@@ -61,7 +61,7 @@
   4. **Session credential return** — what does the hub return on
      successful registration? A `PeerEntry`? A session token? Both?
      How does the returned credential feed into the subsequent
-     `alknet/channels` connection's `CallCredentials`?
+      `alknet/channels` connection's `ConnectionCredentials`?
   5. **Relationship to OQ-58** — the HTTP registration endpoint
      (OQ-58) and `alknet/register` share the enrollment semantics
      (create `PeerEntry`, return credential) but differ in transport