ci: bootstrap required linx PR gate

[WLDFA]

Purpose

Bootstrap the required status-check workflow on linx so subsequent pull requests targeting linx can be validated before merge.

Gate behavior

• resolves the latest commit of LinxISA/llvm-project@dev-llvm15_56 at the start of every run
• builds or restores the cache for that exact LLVM SHA
• runs TileOP contract checks, target compilation, negative cases, PTO MC/disassembly validation, and target C++ frontend validation
• triggers for PRs targeting linx, merge queue groups, pushes to linx for audit, and manual dispatch

This PR only adds .github/workflows/pr-gate.yml; it contains no TileOP source changes.