feat(cli): add omp install target for skill installer

[bnivanov]

What

Adds omp (the Oh My Pi agent CLI) as a fifth op install --target value
alongside claude, codex, cursor, and opencode, with oh-my-pi as an alias.

Why

omp discovers user skills from ~/.omp/agent/skills. Today omp users have to
place the skill bundle manually; a first-class target makes op install
work out of the box and lets bare op install auto-detect omp (via the omp
binary on PATH or an existing ~/.omp directory).

How

• Target::Omp + parse (omp, oh-my-pi) + key + detection.
• Install mirrors the codex/opencode layout: stage the bundle at
  ~/.omp/openpencil-skill, then link ~/.omp/agent/skills/openpencil-skill
  → bundle/skills (copy fallback via the existing link_or_copy_dir).
• The discovery entry is recreated on every install so a stale symlink, plain
  file, or outdated copy can't shadow the fresh bundle (same rationale as the
  opencode target).
• Deliberately not writing to ~/.agents/skills (which omp also scans):
  that path is shared with the codex target and would make uninstall ownership
  ambiguous.
• Uninstall removes the link and the staged bundle.

Tests

• Unit: omp_is_an_install_target (parse, key, alias).
• Integration: install_omp_writes_agent_skills_dir_and_uninstall_removes_it
  — stale-entry replacement, idempotent install, SKILL.md reachable under
  the scanned skills dir, uninstall removes both paths.
• Local: cargo test -p op-cli 108/108 pass; cargo clippy -p op-cli --all-targets -- -D warnings clean; cargo fmt -p op-cli applied.

[Kayshen-X]

Thanks for the contribution! Could you please sync your branch with v0.8.3 and resolve the merge conflicts? Once that's done, we'll take another look.