CP-3580: add coverage gate to squawk-pre-commit

[stephen-kintsugi]

Summary

• Added the standardized coverage gate to CI (lint-and-test job), mirroring kintsugi-rules be-pr.yml but run directly (squawk CI is not Dockerized).
• pytest now produces coverage.xml and enforces a total floor via --cov-fail-under=95 (current total is 97%, so the gate passes today and ratchets later).
• On PRs, diff-cover fails when changed-line coverage vs origin/main is below 80%.
• Added diff-cover ~=10.2.1 dev dep (pytest-cov was already present); gitignored coverage.xml.
• Bumped the package version 0.3.4 -> 0.3.5 in pyproject.toml and squawk_alembic/__init__.py to satisfy the version-consistency gate.

Jira

CP-3580

Tests

Ran locally: pytest --cov-fail-under=95 passes at 97.11% total; diff-cover coverage.xml --compare-branch=origin/main --fail-under=80 passes (changed lines 100%). Pre-commit (ruff, pyrefly, poetry-check) green.

Document Checklist

n/a

🤖 Generated with Claude Code

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Note

Low Risk
Changes are limited to CI, dev dependencies, gitignore, and version bump; no runtime or application logic is modified.

Overview
Adds coverage enforcement to the lint-and-test CI job: pytest now reports coverage for squawk_alembic, writes coverage.xml, and fails below 95% total coverage.

On pull requests, a new step runs diff-cover against origin/main and fails if changed-line coverage is under 80% (with an extra git fetch so the merge-base is available).

Adds diff-cover as a dev dependency, ignores coverage.xml in git, and bumps the package version 0.3.4 → 0.3.5 in pyproject.toml and squawk_alembic/__init__.py.

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