fix(brief): say an archy.yaml was found but invalid, instead of that none exists (#790) - #944

Closes #790.

What and why

archy brief and MCP archy_brief caught LayerConfigError around coverage and set coverage = config = None. load_config accepts overlapping layer patterns and compute_coverage raises on them later, so a config that archy check rejects was shown by brief as no config at all: "(no archy.yaml discovered, so no layer rule governs anything)", and coverage: null with no reason in JSON and MCP.

The catch now keeps the error:

• text: the blind-spot section says archy.yaml found but invalid: <error> instead of the no-config line, and the rest of the briefing still prints;
• JSON and MCP: a new coverage_error field (string or null) beside coverage, on BriefPayload and in archy brief --format json.

A config that does not parse (for example an unknown key) still exits with an error before this point, as before; coverage_error covers the case in the issue, a config that parses but whose layer coverage fails validation. README, the MCP tool description and both copies of the skill (skills/archy/SKILL.md, plugins/claude/skills/archy/SKILL.md) say so.

Proof

New fixture overlapping_layers_project (the repro in #790) first asserts that compute_coverage really raises "multiple layers" on it. Tests: test_brief_reports_invalid_coverage_in_text, test_brief_reports_invalid_coverage_in_json, test_brief_reports_invalid_coverage_in_model_dump. Controls with no archy.yaml and with a valid one now assert coverage_error is null, in CLI JSON and MCP.

Mutation check, tests kept, run on tests/test_cli_contracts.py and tests/test_mcp_brief.py:

With the change, all 32 pass.

On main 0f90188: full pytest 2106 passed, 2 skipped. ruff check, ruff format --check, ty check, archy check ., archy cycles . --strict, archy contracts . and archy conventions . --emit-headers --check all exit 0. The mcp 2.x compatibility job run locally the way ci.yml does it (mcp 2.3.0, no lockfile): 291 passed, 1 skipped. Not run here: the Python 3.10, install and smoke jobs.

Seen, not changed here

The comment above coverage in src/archy/mcp_payloads.py says brief "tolerates a missing or malformed config". A config that does not parse is not tolerated (it raises before coverage, at the parent commit too). I left it alone to keep this PR to #790; happy to fix the wording here or in a separate issue, whichever you prefer.

This pull request was written by Nightlab, an AI agent (https://github.com/nightlab-agent). Before opening it, Nightlab ran /nightlab/prs/archy-790/.venv/bin/python -m pytest -q -p no:cacheprovider tests/test_cli_contracts.py tests/test_mcp_brief.py on this branch and it passed. If AI-written contributions are not welcome here, close it and Nightlab will not open another pull request on this repository.