fix(install): copy skill on opencode always-on install

[michaelxer]

Summary

graphify opencode install wrote AGENTS.md and the OpenCode plugin, but never copied skill-opencode.md to ~/.config/opencode/skills/graphify/SKILL.md, so /graphify was not discoverable.

Linked Issue

Fixes #2670

What Changed

• User-scope install for the always-on branch (aider / codex / opencode / claw / droid / trae / trae-cn / hermes) now calls _copy_skill_file(cmd) before _agents_install, matching amp / agents.
• Matching uninstall removes the skill via _remove_skill_file.
• Help text for opencode / codex install mentions the skill copy.
• Regression tests cover graphify opencode install / uninstall and graphify codex install end-to-end through main().

Testing

python -m pytest tests/test_install.py -q
# 98 passed

python -m pytest \
  tests/test_install.py::test_opencode_cli_install_copies_skill_file \
  tests/test_install.py::test_opencode_cli_uninstall_removes_skill_file \
  tests/test_install.py::test_codex_cli_install_copies_skill_file \
  -q
# 3 passed

Manual check before the fix: graphify opencode install left no ~/.config/opencode/skills/graphify/SKILL.md. After the fix, that path is created and uninstall removes it.

Notes

• Project-scope graphify opencode install --project already went through _project_install, which copied the skill. Only the bare user-scope path was missing it.
• I did not run a live OpenCode binary end-to-end; verification is the install path + unit tests above.