feat: story regression foundation - @pytest.mark.story marker + traceability API (#1699)

[prompt-driven-github]

Summary

Establishes the story-regression foundation for EPIC #1698: a deterministic, marker-traceable pytest oracle that links user_stories/story__*.md files to runnable regression tests via a @pytest.mark.story marker and a bidirectional story↔test traceability API. No test generation yet (that is sub-issue 2).

Closes #1699

Changes Made

Prompts Modified

• pdd/prompts/story_regression_python.prompt (NEW) - deterministic, marker-traceable pytest oracle (pdd/story_regression.py). Builds a bidirectional story_id ↔ {nodeid} map via a pytest --collect-only plugin; exposes tests_for_story, story_for_test, and has_regression_test; detects both orphan directions. No LLM, no test execution.
• pdd/prompts/user_story_tests_python.prompt - promotes the private _slug_from_story_path() slug extractor to a public, documented story_id helper so the marker mechanism and story files share one identity space. Purely additive; LLM-oracle flow unchanged.
• pdd/prompts/coverage_contracts_python.prompt - adds an orthogonal, story-keyed has_regression_test dimension (true when ≥1 collected test claims the story via the marker), computed by querying story_regression.tests_for_story. Existing rule-keyed (test_R<n>) statuses and priority are untouched.

Direct Edits (no originating prompt)

• pytest.ini - registers the story(story_id) marker alongside e2e/integration/slow/real/private_prompt so pytest -m story selects exactly the marked tests with no PytestUnknownMarkWarning.

Documentation Updated

• docs/coverage_contracts.md - new "Story regression coverage" section + Rule-fields note.
• docs/generating_user_stories.md - new "Story regression tests (@pytest.mark.story)" section + Quick-reference row.
• README.md - one-clause coverage-matrix pointer.

Other

• architecture.json - updated to reflect the new story_regression module.

User Stories

• Policy: warn
• user_stories/story__coverage_contracts_story_regression_user_story_tests.md — issue-derived story linked to: coverage_contracts_python.prompt, story_regression_python.prompt, user_story_tests_python.prompt
• user_stories/contracts/coverage_contracts_story_regression_user_story_tests.contract.md — generated machine-checkable contract
• Validation: ❌ 1 of 1 linked story check(s) failed: user_stories/story__coverage_contracts_story_regression_user_story_tests.md
• ⚠️ Warning (non-blocking): failing linked story check(s): user_stories/story__coverage_contracts_story_regression_user_story_tests.md

Review Checklist

• Prompt syntax is valid
• PDD conventions followed
• Documentation is up to date

Next Steps After Merge

1. Regenerate code from modified prompts in dependency order:

   pdd sync coverage_contracts
   pdd sync user_story_tests
   pdd sync story_regression
   

2. Run tests to verify functionality (pytest -m story should select exactly the story-backed regression tests)
3. Deploy if applicable

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Created by pdd change workflow