feat: --github-matrix

[JPHutchins]

Summary

• New --github-matrix flag emits a GitHub Actions matrix as JSON for the task's axes, so the matrix definition lives once in tasks.py instead of being duplicated in workflow YAML.
• Output is the object-of-arrays form ({"PY": ["3.10", "3.11"]}), consumable either as the whole matrix (matrix: ${{ fromJSON(...) }}) or per-axis (PY: ${{ fromJSON(...).PY }}) — the latter lets you combine with extra YAML-side axes like os.
• TTY-aware: indented JSON for interactive preview, compact one-liner for >> $GITHUB_OUTPUT.
• Reuses existing override pipeline — camas matrix --github-matrix --PY 3.13 emits the filtered matrix.
• Mutually exclusive with --dry-run; errors helpfully if the task has no matrix axes; rejects -- passthrough.
• Per-task --help advertises [--github-matrix] when matrix axes exist.

$ camas matrix --github-matrix
{
  "PY": ["3.14", "3.13", "3.12", "3.11", "3.10", "3.15"]
}

$ camas matrix --github-matrix --PY 3.13,3.14 | cat
{"PY":["3.13","3.14"]}

Validation

Emitted JSON is tested against the GHA workflow schema's matrix subset, vendored from SchemaStore (tests/fixtures/github-actions-matrix-schema.json, sha dbc0fd17). Two adversarial tests against the fixture itself (empty axis / empty object) guard against fixture drift.

34 new tests covering pure helpers, schema conformance for six matrix shapes, CLI integration through dispatch, override + mutex + passthrough cases, TTY vs pipe formatting, and per-task help. All 643 existing tests still pass.

Dogfood

The check job's hardcoded python: ["3.10".."3.15"] is replaced by a new discover job that runs camas matrix --github-matrix; the check job consumes ${{ fromJSON(needs.discover.outputs.matrix).PY }}. .python-version becomes the single source of truth. Runner shape unchanged (4 OS × 6 PY = 24).

Test plan

• uv run camas check passes locally (it does — minus pre-existing pyrefly worktree-path issue)
• CI passes — discover job emits matrix; check jobs fan out across the emitted PY list
• camas matrix --github-matrix in a TTY shows indented JSON
• camas matrix --github-matrix | cat shows compact JSON
• camas check --github-matrix fails with "no matrix axes" (since check has no matrix)
• camas matrix --dry-run --github-matrix is rejected as mutex
• Per-task help: camas matrix --help shows [--github-matrix]; camas test --help does not

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8a7d633) to head (cc3d538).

Additional details and impacted files

@@            Coverage Diff             @@
##              main       #17    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           52        54     +2     
  Lines         4773      4977   +204     
  Branches       257       263     +6     
==========================================
+ Hits          4773      4977   +204     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[Copilot]

Pull request overview

Adds a new CLI preview mode (--github-matrix) that emits a GitHub Actions-compatible matrix JSON derived from a task’s declared matrix axes, enabling CI workflows to source matrix definitions directly from tasks.py rather than duplicating them in YAML.

Changes:

• Add --github-matrix CLI flag (mutually exclusive with --dry-run) and dispatch path to emit TTY-aware JSON.
• Introduce camas.main.github_matrix helpers to project matrix axes and format JSON output.
• Add schema-backed tests + vendored schema fixture; update CI workflow to “discover” the Python matrix dynamically.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/camas/main/parser.py	Adds --github-matrix flag and reserves its name from axis overrides.
src/camas/main/dispatch.py	Implements --github-matrix execution path (rejects passthrough, applies overrides, prints JSON).
src/camas/main/github_matrix.py	New helper module to project matrix axes and format/emit JSON.
src/camas/main/format.py	Updates per-task help usage line to optionally mention --github-matrix.
tests/main/test_github_matrix.py	Adds extensive unit + CLI integration tests and schema validation.
tests/fixtures/github-actions-matrix-schema.json	Vendors GHA matrix subschema used for validation tests.
.github/workflows/ci.yaml	Adds discover job and uses emitted matrix for Python versions in check job.
pyproject.toml	Adds jsonschema + types to dev dependencies for schema validation tests.
uv.lock	Locks new dev dependencies (jsonschema, types-jsonschema, and transitive deps).

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