fix(git): carry NIP-OA delegation in auth event

[tlongwell-block]

Summary

• include BUZZ_AUTH_TAG / nostr.authtag in the signed NIP-98 event emitted by git-credential-nostr
• let the relay extract that signed NIP-OA tag for the existing owner-membership fallback
• preserve the legacy x-auth-tag path for direct HTTP clients
• fail closed when a configured auth tag is malformed

Git's credential-helper protocol only returns the Authorization credential; it cannot inject a separate x-auth-tag HTTP header. As a result, managed agents authenticated successfully as themselves but always reached the relay-membership check without their owner attestation.

Verification

• bin/cargo fmt --all -- --check
• bin/cargo check -p buzz-relay
• bin/cargo test -p git-credential-nostr (8/8)
• pre-commit hooks passed
• pre-push non-Rust suites passed; the bare-cargo Rust hook is locally blocked because system rustc 1.89 is below sqlx 0.9's 1.94 MSRV. The same relay check and helper tests pass under the repo's Hermit rustc 1.95.

[tlongwell-block]

The transport mechanism is directionally correct: the auth tag is covered by the verified NIP-98 event signature and verify_auth_tag binds the owner attestation to that event's authenticated agent pubkey. However, this Git admission path accepts syntactically valid timestamp conditions without evaluating them.

buzz_sdk::nip_oa::verify_auth_tag validates the conditions grammar and signature, but does not enforce created_at<t / created_at>t. GitAuth then grants owner-membership fallback without comparing those conditions to the already freshness-checked NIP-98 event's created_at. This means an expired NIP-OA credential continues to grant Git relay admission while its owner remains a member. That conflicts with the admission semantics documented in docs/nips/NIP-AA.md (steps 4 and Auth tag expiry).

Please evaluate timestamp clauses against the verified NIP-98 event timestamp before owner-membership fallback, and add relay-side positive/negative tests (at minimum valid owner delegation, wrong-agent signature, expired/not-yet-valid conditions, and multiple auth tags). The helper tests currently prove serialization/signature coverage, but not the authorization decision.