telemetry(core): trace tool dispatch stages

[apanasenko-oai]

Summary

Make the Core tool-dispatch path attributable before unified exec reaches exec-server.

• Trace parallel-execution locks, lifecycle hooks, handler invocation, policy checks, and approval work.
• Break unified-exec argument parsing, environment selection, command resolution, permissions, and patch interception into stages.
• Preserve dispatch, cancellation, approval, and hook behavior.

This is the Core tool-dispatch tracing slice split out of #30632.

Validation

• Seven focused dispatch, cancellation, unified-exec, hook, and approval tests passed.
• just fmt passed, with an unrelated justfile rewrite discarded.