feat(test): coverage gate + restore ADR-002 (test-strategy pass 1)

[silversurfer562]

Summary

Pass 1 of the test-strategy spec — Layer 1 (attune-help). Closes coverage gaps, adds missing test types, restores ADR-002 (zero required deps beyond python-frontmatter), and lands a CI coverage gate.

• 44 new tests across 5 new files; total 309 passing
• Coverage gate enforced at 81% (branch coverage); current measurement 82.31%
• Circular import unblocked by moving attune-author to the [authoring] extra
• Zero-dep contract under test via real-venv smoke test

What changed

ADR-002 restoration

• Moved attune-author from [project.dependencies] to [project.optional-dependencies.authoring] in pyproject.toml.
• Guarded the 4 deprecated shim modules with try/except ImportError pointing users to pip install attune-help[authoring].
• Added pytest.importorskip(\"attune_author\") to test_manifest.py, test_staleness.py, test_symbols.py, test_authoring_shims.py.

New test files

File	Purpose
tests/test_zero_dep_install.py	Real-venv smoke test guarding ADR-002 (5 tests, @pytest.mark.slow).
tests/test_storage_protocol.py	Reusable StorageProtocolTester mixin verifying the SessionStorage Protocol against LocalFileStorage + an in-memory reference impl (8 tests).
tests/test_templates.py	Direct tests for _find_template_file (incl. CWE-22 guard), _parse_template_file, and the mtime-aware parse cache (12 tests).
tests/test_adapter_rag.py	AttuneHelpAdapter defaults, override paths, protocol conformance (6 tests).
tests/test_mcp_handlers.py	Direct unit tests for _require_str, engine builder, server factory + singleton (13 tests).

Coverage + CI

• pyproject.toml gains [tool.pytest.ini_options] (registered slow marker) and [tool.coverage.*] with branch = true and fail_under = 81.
• tests.yml runs coverage only on the ubuntu-latest x py3.11 cell of the existing matrix; uploads the report as an artifact.
• New test-no-authoring-extra job verifies the importorskip skip path actually works when the extra isn't installed.
• New tests/README.md documents how to run tests and points at the attune-author conftest as the LLM-mocking reference (pass 2 will formalize).

Test plan

• pytest --cov locally — 309 passing, 82.31% branch coverage
• ruff check src/ tests/ clean
• Verified all 5 zero-dep smoke tests pass (real venv creation, ~40s)
• CI matrix passes on ubuntu/macos/windows × py3.10–3.13
• CI test-no-authoring-extra job passes
• Coverage artifact uploads correctly

Rollback

Revert in this order if needed:

1. Drop fail_under line in pyproject.toml (releases the gate without losing tests)
2. Delete the 5 new test files
3. Revert shim try/except guards + test importorskip lines
4. Revert pyproject dep move (restores attune-author as required)

Notes

• The 3 DeprecationWarning outputs in the test run are intentional — the shim modules emit them by design until the 2026-07-07 sunset.
• This is one of four Pass 1 PRs (one per attune layer). The others — attune-gui, attune-rag, attune-author — will land independently.

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