Minor spec hygiene & example correctness (post-split)

[clundie-CL]

Problem description

Small cleanups surfaced during a post-split (#153) holistic review pass. None gate the RC; consolidating so they aren't lost.

1. Write-only fields in success-response examples. Success-response examples include writeOnly fields (e.g. securityMode.password in TrustDomainResponseBasic / TrustDomainResponseWiFiThread) because they reuse request-side YAML anchors. redocly lint flags no-invalid-media-type-examples (this rule is local-only — it is not part of the centralized CAMARA validation pipeline, which is green). Fix by using response-side anchors that omit write-only fields.
2. 404 on a collection endpoint. GET /network-access-devices declares a 404 response; the other collection GETs (/trust-domains, /reboot-requests, /services, /trust-domains/{id}/devices) correctly don't.
3. Shared schema description hardcodes a resource name. ResourceAudit.createdAt / modifiedAt descriptions in NAM_Common.yaml say "the trust domain" though the schema is reused by RebootRequest and TrustDomainDevice.
4. Orphaned examples. Defined but not wired to any operation: TrustDomainDeviceCreateAssignCredential (TrustDomainDevices.yaml), RebootRequestResponseInferred, RebootRequestResponseExplicit (RebootRequests.yaml).

Expected behavior

Spec hygiene fixes per the above; no functional/API-shape change.