feat: migrate from google-generativeai to google-genai SDK

[rosspeili]

Replace deprecated google-generativeai usage with the supported google-genai SDK across optional dependencies, skill runtime code, reference examples, and usage docs. Add PyPI-to-import aliases in SkillLoader so manifest requirements such as google-genai resolve correctly at load time.

Fixes #80

Description

Migrates Skillware off the deprecated Gemini SDK:

• pyproject.toml: [gemini] and [all] extras now install google-genai
• skillware/core/loader.py: map PyPI package names to import paths (google-genai → google.genai, etc.)
• Skills: update Gemini calls in tos_evaluator, mica_module, and synthetic_generator
• Examples: migrate gemini_*.py and mica_rag_flow.py to google.genai.Client() + GenerateContentConfig patterns
• Docs: install note in docs/usage/gemini.md, usage README and agent_loops.md alignment
• Tests: update test_tos_evaluator mocks for the new client API

Verified locally with Python 3.13: pytest tests/ (50 passed), flake8 on changed files.

Also addresses the remaining scope of #96 (examples + complementary usage docs). Recommend closing #96 after merge.

Type of Change (Matches Issue Templates)

• Skill Proposal: New Skill (Contains manifest.yaml, skill.py, and instructions.md)
• Bug Report Fix: Non-breaking change which fixes an execution error or framework bug
• Doc Fix: Documentation Update
• Framework Feature / RFC Updates: Core Framework Update (Changes to base_skill.py, loader.py, etc.)

Checklist (all PRs)

• My code follows the Agent Code of Conduct.
• I have run python -m flake8 . and pytest tests/ locally (or the subset relevant to this change).

New or updated skill

Skipped — no new registry skill. Existing skills were updated as part of the SDK migration only.

Constitution & Safety

No change to skill constitutions. Migration preserves existing behavior: optional Gemini calls remain behind env keys and graceful skip paths when google-genai or GOOGLE_API_KEY is unavailable.

Related Issues

Fixes #80
Also completes the remaining scope of #96