Clarify contributor workflow and harden CI

[Parshan-MHB]

Summary:
Replace the stuck contributor-workflow PR with a fresh branch from current main and harden CI so package tests run in an isolated scratch path with a bounded timeout.

What Changed:

• clarify that external contributors should use forks while maintainers use feature branches in the main repository
• document the practical permission boundary behind that workflow
• update CI to run swift test with a scratch path and a job timeout

Verification:

• swift test --scratch-path /tmp/APTerminalCIReplacement

Security Impact:
Repository governance only. CI isolation improved. No runtime behavior change.

Screenshots:
N/A