fix(cli): report presence subject

[tlongwell-block]

What

Make buzz users presence report the presence subject from the first well-formed p tag, falling back to the event author when that tag is absent or lacks a value.

The relay's /query bridge synthesizes presence events signed by the relay and places the actual user in a p tag. The CLI currently reports the relay signer, so every row can appear to belong to the same pubkey.

This matches the desktop extraction rule without adding hex validation. A bare ["p"] tag falls back to the author because it has no subject value.

Scope

One production-line substitution plus a 14-line extractor. Three focused regression tests cover:

• relay-synthesized event: p-tag subject wins
• direct user-signed event: author fallback
• malformed bare p tag: author fallback

Validation

• cargo test -p buzz-cli — 150 passed
• cargo clippy -p buzz-cli --all-targets -- -D warnings
• cargo fmt --check
• release build: cargo build --release -p buzz-cli
• full pre-push suite under repo Hermit/Rust 1.95 — passed
• live staging probe with the release binary:
  • queried Eva + Mari
  • returned each requested subject pubkey
  • did not return relay signer 79be667e…f81798

Testing followed root TESTING.md, crates/buzz-cli/TESTING.md, and the Buzz testing skill. Because this is a pure CLI response-mapping change, I used its prescribed scoped live probe rather than a destructive relay clean-room deployment.

[tlongwell-block]

Perci review: APPROVE (no blocking findings). Reviewed commit 69a71b849. The extractor matches desktop semantics (first p tag with a string subject, otherwise author), the three regression cases cover synthesized/direct/bare-tag paths, and focused tests plus an independent live staging probe passed. GitHub would not record a formal approval because the shared CLI identity is the PR author.