fix: validate stale submissions with current trusted code

[lessthanno]

What

• Check out the current official base repository and base branch for the trusted validator.
• Keep participant branches out of the trusted checkout.
• Extend the workflow contract test to prevent a return to the stale base SHA.

Why

Older open submissions such as #33 and #35 still point to a base commit created before the current validator existed. Reopening those PRs therefore repeats the old repository-side failure instead of validating the submission.

Test plan

RUN v4.1.5 /Users/xiaozihao/Documents/01_Projects/Team_AI/xagt-plugin

Test Files 1 passed (1)
Tests 38 passed (38)
Start at 17:10:17
Duration 370ms (transform 44ms, setup 0ms, import 73ms, tests 125ms, environment 0ms)

@xagt/agent-plugin@0.5.0 lint
tsc -p tsconfig.json --noEmit

@xagt/agent-plugin@0.5.0 test
vitest run

RUN v4.1.5 /Users/xiaozihao/Documents/01_Projects/Team_AI/xagt-plugin

Test Files 10 passed (10)
Tests 90 passed (90)
Start at 17:10:19
Duration 761ms (transform 384ms, setup 0ms, import 745ms, tests 795ms, environment 1ms) — 10 files, 90 tests passed

@xagt/agent-plugin@0.5.0 build
tsc -p tsconfig.json

Risk

The trusted checkout remains explicitly bound to the official PR base repository and branch, uses read-only permissions, and does not persist credentials. Participant code is neither checked out into the trusted path nor executed. This PR does not change submission eligibility or judging rules.