feat: add oh-my-pi (omp) as a supported agent

[oldschoola]

Summary

Adds oh-my-pi (@oh-my-pi/pi-coding-agent, binary omp) as a new agent integration, parallel to the existing pi (@earendil-works/pi-coding-agent) hook. They are different projects that happen to share a name and an ExtensionAPI shape; the existing pi integration is left fully intact.

• New TypeScript extension at hooks/omp/rtk.ts — delegates to rtk rewrite on the tool_call event via the omp ExtensionAPI, fail-open, version-guarded rtk >= 0.40.0.
• New AgentTarget::OhMyPi clap variant: rtk init --agent omp (alias --agent oh-my-pi). Install paths: ~/.omp/agent/extensions/rtk.ts global (honors the same PI_CODING_AGENT_DIR env that oh-my-pi itself reads) or .omp/extensions/rtk.ts project-local.
• run_omp_mode / uninstall_omp wired through run_cli and uninstall_init_dispatch alongside Pi and Hermes; doc updates in top-level README.md and hooks/README.md.

Test plan

• cargo build clean
• cargo test --bin rtk — 2003 passed, 0 failed (1997 baseline + 12 new mirrored install/uninstall/dry-run tests for both global and local scope + 3 new CLI parse tests for --agent omp, the oh-my-pi alias, and the uninstall form)
• Manual smoke: rtk init --agent omp --dry-run and rtk init --agent oh-my-pi --dry-run both emit the expected "would create .omp/extensions/rtk.ts" plan; rtk init --help lists the new variant
• cargo fmt --all && cargo clippy --all-targets — not run on this Windows workstation; happy to run if CI flags anything

Notes

• pi (earendil) integration is completely untouched — no shared code paths were modified, only new symbols added.
• omp reuses the existing PI_CODING_AGENT_DIR env var because the oh-my-pi runtime itself reads that same variable (different default base: .omp/agent vs .pi/agent). Documented in hooks/omp/README.md.
• hooks/omp/rtk.ts mirrors hooks/pi/rtk.ts line-for-line aside from the import source and the minimum-version constant — keeps future cross-maintenance trivial.

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