feat(compile): add PR trigger filters with pre-activation gate

Add triggers.pr front matter field with native ADO branch/path config
and runtime filter evaluation via gate steps in the Setup job.

Phase 1 (Tier 1 filters):
- PrTriggerConfig, PrFilters, PatternFilter, IncludeExcludeFilter,
  LabelFilter, BranchFilter, PathFilter types in types.rs
- Gate step generation with title, author, source-branch, target-branch
  filters using ADO pipeline variables
- Self-cancel via ADO REST API when filters don't match (cancelled builds
  are invisible to DownloadPipelineArtifact, preserving memory chain)
- Build tag diagnostics (pr-gate:passed, pr-gate:skipped,
  pr-gate:<filter>-mismatch) and task warnings for filter failures
- Agent job condition: non-PR builds bypass gate, PR builds require
  prGate.SHOULD_RUN=true
- triggers.pr overrides schedule/pipeline trigger suppression
- Native ADO pr: block emitted for branch/path filters

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jamesadevine