## Why
Plugin and skill loading is useful as warmup and early validation, but
session startup does not need to wait for that work before it can
continue building the session. Keeping it on the serial startup path
adds avoidable latency to every fresh thread start.
We still want invalid skill configurations to show up quickly, and we
want the warmup to exercise the same plugin and skill manager caches
that the normal turn path uses.
## What changed
- moved plugin and skill warmup into the session startup async path
instead of eagerly awaiting it on the serial setup path
- kept the warmup using the session's resolved filesystem/environment
context so skill loading still sees the right roots
- preserved early skill-load error logging so broken skill
configurations still surface during startup
- left the per-turn plugin and skill loading path unchanged, so turns
still use the normal cached managers
## Testing
- Not run locally; relying on CI for validation.
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