fix(telemetry): gate preamble session tracking

[jbetala7]

Fixes #1150

Summary

• gate preamble session-file tracking, pending-marker handling, and timeline logging behind telemetry != off
• move spawned-session detection behind a helper so generated SKILL.md files no longer embed the OPENCLAW_SESSION string directly
• update README wording and add regression tests around telemetry gating and generated skill output

Root cause

The preamble generator still emitted several local tracking side effects unconditionally even though telemetry: off is documented as disabling local analytics, and it embedded the OPENCLAW_SESSION env var name directly into every generated SKILL.md preamble.

Testing

• bun test test/telemetry.test.ts test/gen-skill-docs.test.ts