[Windows port] 8/8: De-bash the docs and skill prose

[wmaciel]

Part of #7. Phase 8 of 8.

Update the remaining shell-shaped prose in the skill docs and user-facing docs so Windows users are not handed bash idioms. Closes #7.

Changes: shadow-frog-dream/SKILL.md prose, README.md, claude.md, agent-context.md, CHANGELOG.md.
Acceptance: CI green on both OSes; no residual .sh/python3 references outside install.sh.
Depends on: Phase 7.

Drafted with Copilot 🤖