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/crates/tls/README.md

@@ -696,7 +696,7 @@ alknet-core (loses TLS setup code + endpoint)
 
 alknet-call (pure protocol crate — no TLS/transport deps per ADR-089 §5)
 └── alknet-core (ProtocolHandler, Connection, types; ConnectionCredentials/
-    RemoteIdentity moved to core per ADR-091; CallCredentials stays in
+    RemoteIdentity moved to core per ADR-091; CallCredentials removed per ADR-091 Am. 2026-07-17
     alknet-call)
 
 alknet-hub (multi-transport endpoint)