feat: add mix ci task for the complete quality gate

[bougyman]

Summary

• Adds Mix.Tasks.Ci (lib/mix/tasks/ci.ex) that runs the four quality gate steps (deps.get, format --check-formatted, usage_rules.sync --check, mix test) sequentially inside app/, failing fast on the first error
• Updates .github/workflows/ci.yaml to call mix ci instead of the four individual steps — CI and local mix ci now share one code path
• Adds test/mix/tasks/ci_test.exs verifying the correct sequence of commands is invoked (using an injected shell runner, matching the pattern from lc_test.exs)

Closes CRY-49

Test plan

• mix compile --warnings-as-errors passes
• mix format --check-formatted passes
• mix test passes (12/12, including the new Mix.Tasks.CiTest)
• CI workflow updated to call mix ci with working-directory: . (repo root), overriding the job's app/ default

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[bougyman]

Code Review

Issue: CRY-49 — Add mix ci task for the complete quality gate
Branch: CRY-49-mix-ci-task
Commit: d703eaf feat: add mix ci task for the complete quality gate

Quality suite

All checks pass on the branch as-is:

• Compilation (warnings-as-errors): clean
• mix format --check-formatted: clean
• mix test: 12/12 passed

Assessment: Approve

Clean, well-scoped implementation that follows the established repo-management task pattern exactly. The code is correct, the CI workflow update is sound, and the test covers the right contract.

What looks good

• Pattern consistency: ci.ex follows the same run/1 (impl) → run/2 (testable, injected shell) pattern as lc.ex. Dependency injection via the shell function argument keeps the test fast and side-effect-free.
• Moduledoc quality: Documents the exact step list and the reason each step exists (including the #79 drift-detection cross-reference). This is the kind of "why" documentation that actually helps.
• CI workflow update: Correctly overrides the job-level working-directory: app default with working-directory: . for the mix ci step, since the task lives in the root project. The consolidation comment preserves the #79 reference.
• Scope discipline: deps.get inside the task makes it self-contained for local use; CI's cache step still runs before it so the deps.get is a no-op when the cache hits — no wasted time.
• Fail-fast behavior: Shell.run!/3 raises on non-zero exit, so the first failing step aborts the gate. Correct for a quality gate.

Issues

Minor

1. Test name overpromises on ordering (test/mix/tasks/ci_test.exs:6)
   The test is named "runs all quality gate steps in order" but assert_receive with distinct patterns doesn't actually verify ordering — it scans the full mailbox for any matching message, so it would pass even if the steps ran in reverse. The test does verify that all 4 steps are invoked with the correct arguments and options, which is the important contract. The ordering claim is technically misleading, though in practice the synchronous implementation guarantees order by construction. Consider either:

   • Renaming to "invokes all quality gate steps with correct arguments", or
   • Collecting messages into a list and comparing with == to actually assert order

   Not a blocker — the implementation's ordering is structurally guaranteed by the sequential shell.() calls.

No issues found

• Correctness: The 4 steps match the prior CI workflow exactly (deps.get, format, usage_rules.sync, test). No steps missing, no extras added.
• Safety: Shell.run!/3 handles non-zero exits. No user input flows through unsanitized.
• Performance: No regressions — deps.get is idempotent and fast when already fetched.
• Conventions: Conventional commit message, proper @impl/@doc false annotations, no unnecessary code.