fix(compile): use macro form for same-job synthPr gate refs

[Copilot]

Summary

The synthetic-PR gate was silently bypassed on real CI builds. prGate runs in the Setup job alongside the synthPr step that emits AW_SYNTHETIC_PR* outputs, but the gate's env: read them via the same-job runtime expression $[ variables['synthPr.AW_SYNTHETIC_PR'] ] — which ADO resolves to empty (step outputs aren't exposed to runtime expressions in the producing job). gate/bypass.ts therefore always took the "not a PR build — pass automatically" branch.

Changes in compile_gate_step_external (src/compile/filter_ir.rs):

• Macro form for same-job synth refs — reference synthPr outputs via $(synthPr.X), which expands from the job's output-variable namespace (empty when the step was skipped/unmatched, never a stray literal):

  AW_SYNTHETIC_PR: "$(synthPr.AW_SYNTHETIC_PR)"
  ADO_PR_ID:        "$(System.PullRequest.PullRequestId)$(synthPr.AW_SYNTHETIC_PR_ID)"
  ADO_SOURCE_BRANCH:"$(System.PullRequest.SourceBranch)$(synthPr.AW_SYNTHETIC_PR_SOURCEBRANCH)"
  ADO_TARGET_BRANCH:"$(System.PullRequest.TargetBranch)$(synthPr.AW_SYNTHETIC_PR_TARGETBRANCH)"

• Mutually-exclusive concatenation replaces coalesce(...) for the branch/id vars: synthPr only runs on non-PR builds and System.PullRequest.* is empty on non-PR builds, so exactly one side is ever non-empty. (Runtime expressions $[ ] must occupy the whole value and can't be concatenated, so the old coalesce form couldn't carry the macro.)
• Cross-job exec-context wiring in the Agent job (dependencies.Setup.outputs['synthPr.X']) is correct and left unchanged.
• Updated the doc comment and the regression guard test_pr_filter_synth_mode_gate_step_uses_same_job_synth_ref to assert the macro form and reject both the variables['synthPr.X'] and cross-job forms inside the Setup-job gate step.

Test plan

• cargo test — full suite green; updated synth gate-step regression test
• Manual: compiled pr-filter-tier1-agent.md and confirmed the Setup-job prGate env emits the macro form and the output YAML parses