fix: isolate oh-my-posh prompt config per theme

[Ansub]

Summary

• stop using a single shared ~/.config/ohmyposh/boo.omp.json as the only active prompt config
• generate/update per-theme prompt configs at ~/.config/ohmyposh/boo..omp.json
• export BOO_OMP_CONFIG in theme.zsh so zsh can load the active theme prompt config
• add shell-side config resolution fallback order: explicit env -> themed file -> legacy file
• keep legacy compatibility by syncing boo.omp.json to the active themed config
• remove both legacy and themed OMP config files during uninstall

Why

Theme-specific prompt edits were bleeding across themes because all themes patched the same shared OMP JSON file.

Validation

• bash -n bin/boo
• zsh -n shell/boo.zsh
• isolated HOME repro confirmed distinct boo.clay.omp.json and boo.abyss.omp.json values