feat(relay): log process id at startup

[sw-square]

What

Add the process id as a structured field on the relay's "Starting buzz-relay" startup log:

info!(pid = std::process::id(), "Starting buzz-relay");

Why

Operators can now correlate the running relay process with signals, log lines, and metrics directly from its first startup log line — no ps/pgrep needed.

Notes

I targeted buzz-relay (the long-running "main entry point" per AGENTS.md) rather than the buzz CLI: the CLI emits structured JSON on stdout, so printing a PID there would corrupt machine-readable output. The relay already logs a startup banner via tracing, so this rides on the existing structured-logging convention — one-line change, no new output surface.

Testing

cargo check -p buzz-relay passes; pre-push gate (fmt, clippy, rust-tests, desktop-tauri-checks) green.

🤖 Generated with Claude Code

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is e17cdd9d5c7e2b836b4670ae88bb87a79f94337a...7dda85dcb70b18bd4f0f6cbdb81b3a8ea5b195e1.
A new review must complete for this exact range. When manual authorization
is required, a Block organization member must comment exactly
@buzz-security-review 7dda85dcb70b18bd4f0f6cbdb81b3a8ea5b195e1 to authorize a new review.
Any previous review applies only to its recorded range.