feat: add opt-in integration test suite (#23)

[cmeans-claude-dev]

Summary

• New tests/test_integration.py with 6 tests exercising real clipboard tools
• Pytest marker integration configured in pyproject.toml -- skipped by default
• Run with uv run pytest -m integration (requires clipboard daemon)
• Tests cover: plain text round-trip, unicode, multiline, special characters, format listing, unavailable MIME type
• Updated CONTRIBUTING.md and CLAUDE.md with integration test commands
• Verified passing on Wayland (Fedora 43)

Test plan

• uv run pytest -q -- 450 passed, 6 deselected, 8 xfailed (integration skipped)
• uv run pytest -m integration -v -- 6 passed (on a machine with clipboard)
• CI passes (integration tests are skipped in CI)

Closes #23

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[cmeans]

Applying Ready for QA Signoff. All 3 checkboxes verified — including running the 6 integration tests against the real system clipboard on Wayland/Fedora 43 (all passed). Default suite correctly deselects them. 450/450 pass. Zero findings.

[cmeans]

QA Review — PR #57

Verdict: Zero findings. Ready for maintainer signoff.

Checkboxes verified

CHECKBOX 1: pytest -q (default)       → 450 passed, 6 deselected, 8 xfailed (matches PR body)
CHECKBOX 2: pytest -m integration -v  → 6/6 PASSED (real clipboard, Wayland/Fedora 43)
CHECKBOX 3: CI passes                 → All green (lint, typecheck, test matrix, codecov)

All checkboxes ticked. Integration tests run against real clipboard on this machine — not just checked for skip behavior.

Code review

Marker config: addopts = "-m 'not integration'" in pyproject.toml skips integration by default. Opt-in with -m integration. Standard pytest pattern. ✓

Test coverage (6 tests):

• Plain text round-trip, unicode (emoji + CJK), multiline, special chars (pipes, quotes, angles, ampersands), format listing after write, empty MIME returns empty
• All use the public API (write_clipboard/read_clipboard/list_clipboard_formats) — exercises real code paths ✓
• .strip() on read results handles platform-specific trailing newline differences ✓
• pytestmark = pytest.mark.integration applies to all tests in the file ✓

Doc updates: CONTRIBUTING.md dev setup block and integration test paragraph, CLAUDE.md commands section — both accurate, both reference the -m integration flag. CONTRIBUTING.md ties it to #5 and #10 (platform testers). ✓

Other checks

• CHANGELOG: New ## [Unreleased] / ### Added — correct (v2.1.1 just released, new feature). Accurate. Closes #23. ✓
• CI: All green. Integration tests correctly deselected (6 deselected visible in default run). ✓
• Diff scope: 5 files, +96/-2, single commit. Clean. ✓

Findings

None.

[cmeans]

LGTM