docs: enable missing_docs across remaining 8 workspace crates

[jewoos2921]

Summary

Completes the workspace-wide missing_docs effort. Combined with #3469 and #3500, every workspace crate now enforces #![warn(missing_docs)].

Crates documented (~600 items, 684 insertions)

Crate	Items	Key areas
buzz-pair-relay	3	Relay struct + constructor
buzz-media	55	MediaError variants, bucket_index/storage fields
buzz-agent	127	types.rs enums/structs/methods, auth.rs/config.rs APIs
buzz-relay-mesh	192	mesh traits, peer/endpoint/gossip/wire/status APIs
buzz-push-gateway	223	apns/app_attest/authority/grant/model/token APIs
buzz-conformance	0	Attribute only — already fully documented
git-credential-nostr	0	Attribute only
git-sign-nostr	0	Attribute only

Also fixed several pre-existing broken intra-doc links surfaced during the doc build.

Verification

• RUSTDOCFLAGS="-W missing_docs" cargo doc — 0 missing warnings across all 8 crates
• cargo clippy -- -D warnings — clean
• cargo fmt --check — clean

Workspace-wide result

After #3469 + #3500 + this PR, all 23 workspace crates enforce missing_docs:

buzz-agent ✅    buzz-acp ✅*      buzz-audit ✅     buzz-auth ✅
buzz-cli ✅*      buzz-conformance ✅ buzz-core ✅    buzz-db ✅
buzz-dev-mcp ✅*  buzz-media ✅      buzz-pair-relay ✅ buzz-persona ✅*
buzz-pubsub ✅   buzz-push-gateway ✅ buzz-relay ✅   buzz-relay-mesh ✅
buzz-sdk ✅      buzz-search ✅     buzz-test-client ✅ buzz-workflow ✅
buzz-ws-client ✅*  git-credential-nostr ✅  git-sign-nostr ✅

(* = added in #3469 or #3500)

[jewoos2921]

Closing — removing this PR.