improve: fix CI branch target (main->master) and fix CONTRIBUTING.md typos

[Coding-Dev-Tools]

Automated improvement by dev-engineer

• Fix CI workflow branch target from main to master (default branch is master, so CI was never triggering on pushes/PRs)
• Fix CONTRIBUTING.md: remove stray backslash before ruff commands, clean up numbered list formatting

[github-actions]

🤖 Automated Code Review

✅ Ruff Lint — No issues

⚠️ Ruff Format — Formatting needed

Would reformat: src/deadcode/__main__.py
Would reformat: src/deadcode/cli.py
Would reformat: src/deadcode/scanner.py
Would reformat: tests/test_cli_edge_cases.py
Would reformat: tests/test_config_and_fixes.py
Would reformat: tests/test_scanner.py
6 files would be reformatted, 3 files already formatted

✅ Secret Detection — Clean

✅ Large Files — Within limits

📊 Diff Stats — 2 file(s) changed

 .github/workflows/ci.yml | 4 ++--
 CONTRIBUTING.md          | 7 ++++---
 2 files changed, 6 insertions(+), 5 deletions(-)

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Verdict: ⚠️ Warnings — Lint/format issues found. Recommend fixing before merge.

Automated by Coding-Dev-Tools/.github reusable workflow.