Add FixerAgent fixtures and pytest tests (triage + diff_generator)

feat(srs): add CI syntax invariants, failure semantics, and agent contract

trigger CI with actual diff

trigger SRS CI

fix(ci): prevent fail-abort by adding continue-on-error to markdownlint and safe-fail guard

trigger CI after indentation fix

fix(ci): correct YAML indentation for artifact steps

trigger SRS CI after artifact path fix

fix(ci): correct absolute artifact path for FixerAgent triage

test: FixerAgent triage after guaranteed artifact

fix(ci): ensure guaranteed srs-ci-logs artifact for FixerAgent triage

test: FixerAgent triage with guaranteed artifact

fix(ci): ensure guaranteed artifact for FixerAgent triage

test: trigger FixerAgent triage with final artifact path

fix(ci): ensure stable logs artifact path for FixerAgent