[P2] Update CI failure triage playbook URLs to existing documentation

[stranske]

Why

In tools/ci_failure_triage.py lines 79, 93, and 107, DEFAULT_TRIAGE_PATTERNS defines playbook_url fields pointing to non-existent documentation sections. The file docs/INTEGRATION_GUIDE.md does not exist in the repository, while docs/CI_SYSTEM_GUIDE.md is the active documentation file. This causes automated CI failure triage comments to generate broken 404 links for maintainers.

Scope

• Update DEFAULT_TRIAGE_PATTERNS in tools/ci_failure_triage.py to reference valid documentation paths in docs/CI_SYSTEM_GUIDE.md.
• Add test coverage in tests/test_main.py verifying all playbook_url references resolve to existing files in docs.

Non-Goals

• Modifying triage regex patterns or suggested fix templates in tools/ci_failure_triage.py.
• Creating redundant duplicate documentation files.
• Scaffold-only completion does NOT count: setting playbook_url to None instead of linking to existing documentation in docs/CI_SYSTEM_GUIDE.md is a failure of this issue.

Tasks

• Update playbook_url paths in DEFAULT_TRIAGE_PATTERNS in tools/ci_failure_triage.py to point to docs/CI_SYSTEM_GUIDE.md
• Add a regression test in tests/test_main.py asserting all non-empty playbook_url paths in DEFAULT_TRIAGE_PATTERNS exist on disk

Acceptance Criteria

• pytest tests/test_main.py passes with all tests green.
• All playbook_url values in tools/ci_failure_triage.py point to existing repository files under docs.
• Deliberate-break demonstration: pointing a playbook_url to a non-existent path causes pytest tests/test_main.py to fail, and reverting restores pass.

Implementation Notes

References: Code/Audits/Ready/2026-09-07-00-repo-map.md.
Verify docs/CI_SYSTEM_GUIDE.md exists and contains relevant triage sections.