refactor(compile): extract PR filters to dedicated module

Move PR trigger filter logic from common.rs to pr_filters.rs:
- generate_native_pr_trigger(), generate_pr_gate_step(), shell_escape(),
  add_condition_to_steps() and all associated tests
- Add Tier 2 filter generators: labels (any-of/all-of/none-of), draft
  (isDraft check), changed-files (iteration changes API + fnmatch)
- REST API preamble only emitted when Tier 2 filters are configured
  (has_tier2_filters() helper)
- 12 new Tier 2 tests (27 total in module)

common.rs reduced by ~450 lines, now delegates to pr_filters::.

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

Author: jamesadevine