feat: align Krita install staging and skill guidance

[loonghao]

Summary

• stage the Krita plug-in tree through Core's lock-aware safe_replace_tree helper with a compatibility fallback
• document the agent-first install lifecycle and receipt-owned uninstall in the bundled skill
• add regression coverage for Core staging delegation and lifecycle guidance

Verification

• python -m pytest -q (59 passed, 2 skipped)
• python -m ruff check src tests tools bridge
• python -m ruff format --check src tests tools bridge
• python tools/lint_skills.py
• python -m py_compile bridge/krita-plugin/dcc_mcp_krita/init.py bridge/krita-plugin/dcc_mcp_krita/runtime.py
• python -m build
• python -m twine check dist/*

Live Krita was not available; no licensed live-host readiness is claimed. Core catalog work remains out of scope (dcc-mcp-core#2252/#2382).