feat: add refactoring advisor skill

[wusuiling-if]

Summary

Add the refactoring-advisor skill requested in #85. The skill audits functions, classes, modules/packages, and cross-cutting dependencies, then produces evidence-backed refactoring proposals without modifying implementation code.

The workflow:

• separates discovery, root-cause diagnosis, proposal design, and prioritization
• requires exact paths, symbols, and line ranges for findings
• requires an explicit dependency injection decision for every proposal
• prefers explicit dependency injection when otherwise-comparable designs exist
• includes language-specific discovery guidance for Java, Python, TypeScript, and C#
• provides a named smell and strategy catalog plus a fixed implementation-handoff report template
• registers the skill in the large-codebase marketplace and generated skills catalog
• blocks executable pseudocode and implementation-ready replacements

Closes #85

Validation

• npm run build:skills
• python3 scripts/sync_extensions.py --check
• python3 -m pytest -q tests/test_refactoring_advisor.py tests/test_skills_catalog.py tests/test_sync_extensions.py tests/test_plugin_manifest.py tests/test_skill_plugin_loading.py tests/test_skills_have_readme.py -k 'not iterate_loads_as_sdk_plugin' (169 passed, 1 deselected)
• focused acceptance checks for trigger coverage, language coverage, output-only behavior, required references, DI evaluation, report structure, and the under-500-line requirement

The full Python suite was not available locally because this machine has Python 3.10 and no uv, while the project requires Python 3.12 and the test group includes openhands-sdk, mcp, and other unavailable dependencies. The repository sync check also reports the pre-existing non-blocking marketplace warning for plugins/issue-duplicate-checker.