telemetry(core): trace session startup stages

[apanasenko-oai]

Summary

Break Core session initialization into attributable stages.

• Trace rollout setup, telemetry creation, git probing, shell resolution, environment setup, AGENTS refresh, and skills warmup.
• Carry trace context into asynchronous environment resolution.
• Preserve startup ordering and configuration behavior.

This is the Core session-startup tracing slice split out of #30632.

Validation

• Five focused session initialization and environment-resolution tests passed.
• just fmt passed, with an unrelated justfile rewrite discarded.