[DEFER] Add opt-in trace mode for lifecycle actions

[mark-e-deyoung]

Summary

Add optional trace mode for long-running lifecycle actions to improve diagnostics without high default overhead.

Scope

• Introduce opt-in trace spans/timing for stage internals.
• Keep default logging lightweight; no always-on high-volume trace output.
• Ensure traces are machine-readable and correlated with run_id.

Acceptance criteria

• Trace mode can be toggled by env/flag.
• Normal mode performance and verbosity remain unchanged.
• Documentation explains when to use trace mode and how to collect artifacts.