feat(hermes): add initial manual plugin support

[HamsteRider-m]

Summary

• add initial Hermes plugin files under hermes/
• accept rtk rewrite exit code 3 as a valid rewrite path
• skip already-rewritten / RTK_DISABLED=1 commands
• add README + hooks docs for initial Hermes support and manual install
• add Python tests for rewrite behavior and Hermes-specific guards

Notes

• this is intentionally based on the latest active branch head rather than the older PR feat: add Hermes agent plugin for RTK command rewriting #1006 branch
• Hermes support is manual for now; rtk init is not wired up yet
• README now explicitly mentions Hermes initial support

Test Plan

• uv run --with pytest python -m pytest hermes/test_rtk_plugin.py -q
• python3 -m py_compile hermes/__init__.py hermes/test_rtk_plugin.py
• manual validation: rtk rewrite 'git status' emits rtk git status with exit code 3 on current builds

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