fix(init): install Pi extension under .omp so OMP discovers it

[manateeJr]

Problem

rtk init --agent pi installs the TypeScript extension to .pi/extensions/rtk.ts (project) / ~/.pi/agent/extensions/rtk.ts (global). The current Oh My Pi (OMP) runtime auto-discovers extensions only from <cwd>/.omp/extensions and ~/.omp/agent/extensions; .pi/extensions is not a discovery root. The extension is therefore written but never loaded, so no commands are rewritten and rtk gain shows no OMP activity.

Root cause

The install directory literals — not the extension code. OMP still honors the PI_CODING_AGENT_DIR env var, and its default agent dir is ~/.omp/agent, so pointing the paths at .omp aligns install, uninstall, and OMP's discovery.

Fix

• src/hooks/constants.rs: PI_DIR .pi/agent → .omp/agent, PI_LOCAL_DIR .pi → .omp
• src/hooks/init.rs: matching doc comments + test assertions
• docs: hooks/README.md, hooks/pi/README.md, src/hooks/README.md, docs/guide/getting-started/supported-agents.md

The extension file (hooks/pi/rtk.ts), the --agent pi flag, the PI_* constant names, and PI_CODING_AGENT_DIR are unchanged. Paths-only, minimal.

Verification

Verified end-to-end against a live omp v17.0.6:

• With the extension at ~/.omp/agent/extensions/rtk.ts, OMP loads it on startup via native discovery and its tool_call handler rewrites bash commands through rtk rewrite.
• Example: ls -la /tmp/… was rewritten to rtk ls -la /tmp/…, executed, and logged in rtk's history at 94.4% savings — rtk gain now reflects OMP usage.

Gate: cargo fmt --all --check, cargo clippy --all-targets, and full cargo test all pass (the test_run_pi_mode_* / test_pi_* tests now assert .omp).

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