Make uploaded skill and local runner boundaries explicit

[Guanzhw]

Two local boundaries surfaced while importing and testing a reusable user skill:

1. When an uploaded user skill and a project skill share a name, a chat can resolve the project copy even though the library entry points at the upload. A new or steered turn should use the verified session copy or explicitly report a source collision, so the user can tell which instructions were loaded.
2. The local experiment launcher serializes synced environment values into run.sh. Those values can include credentials. The script should contain the runnable payload while the child receives environment values through its process environment.
3. A local experiment timeout outside the supported range should be rejected when the command is accepted, before a run is created.

I reproduced the skill-source mismatch during a ZIP import pilot, and the launcher issue during local experiment runs. The proposed fix and tests are in a draft PR; private values and raw local scripts are excluded.