telemetry(exec-server): trace local process lifecycle

[apanasenko-oai]

Summary

Trace the local exec-server child-process lifecycle independently from transport and remote-client setup.

• Separate stdout, stderr, exit, and close lifecycle spans.
• Record first output, receiver wait, process-lock wait, notification lookup, and publication timing.
• Preserve output retention, exit classification, and close semantics.

This PR is stacked on the exec RPC foundation in #30675 and is split out of #30632.

Validation

• Six focused output retention, exit, termination, close, and trace-export tests passed.
• just fmt passed, with an unrelated justfile rewrite discarded.
• The broader exec-server suite remains environment-limited by macOS sandbox-exec permission denials and local network or HTTP timeouts.