telemetry(core): trace unified exec process lifecycle

[apanasenko-oai]

Summary

Make the Core unified-exec process lifecycle attributable.

• Attribute remote attachment, pushed terminal events, recovery reads, output collection, and late denial handling.
• Record first output, exit, close, recovery, and completion milestones.
• Add a deterministic regression seam for early remote sandbox denial.

This is the Core unified-exec lifecycle slice split out of #30632.

Validation

• Six focused remote exec, early denial, output, pause, and process-completion tests passed.
• just fmt passed, with an unrelated justfile rewrite discarded.