feat(omp): add extension-based rewrite integration for Oh My Pi

[makoMakoGo]

Summary

Adds Oh My Pi (OMP) as a first-class RTK agent target via a runtime
extension that intercepts tool_call events for the bash tool and
rewrites the command through rtk rewrite.

• New agent: rtk init --agent omp / rtk init -g --agent omp
• Installs a self-contained TS extension to ./.omp/extensions/rtk.ts
  (project) or ~/.omp/agent/extensions/rtk.ts (global)
• Extension is a thin delegate — all rewrite logic stays in the Rust
  registry (src/discover/registry.rs); the TS file only dispatches
  rtk rewrite and applies the result
• Auto-rewrite on Allow / Ask-Default; Deny still blocks. Matches the
  opencode plugin's precedent of "rewrite = allow" rather than
  replicating Claude's exit-3 ask semantics in-agent
• Startup check: if rtk is not on PATH at load time, the extension
  registers no handler and stays silent (same pattern as the opencode
  plugin's await \$\which rtk``)
• --agent omp is mutually exclusive with --codex, --gemini,
  --opencode, --claude-md, --hook-only, --auto-patch,
  --no-patch, --copilot (validated in validate_init_target_selection)
• Idempotent install: stock content is detected and not rewritten;
  pre-existing non-stock files are refused rather than clobbered
• --show and --uninstall both supported and scoped to the stock
  extension marker

Closes #591.

Review changes applied

Two rounds of review before opening:

1. Initial self-review against the OMP source at
   packages/coding-agent/src/extensibility/extensions/ to verify
   tool_call dispatch, ToolCallEventResult block semantics, and
   BashToolInput.command shape
2. Cross-check against peer hook implementations
   (hooks/claude/rtk-rewrite.sh, hooks/cursor/rtk-rewrite.sh,
   hooks/opencode/rtk.ts) to avoid over-engineering relative to
   established RTK conventions. Peers have no spawn timeout, silent
   catch {}, and no TS tests — so those suggestions were dropped. The
   surviving fix was the startup PATH check (from opencode) replacing
   the per-call `rtkMissing` latch.

Test plan

• `cargo fmt --all` clean
• `cargo clippy --all-targets` — no new warnings
• `cargo test test_omp` — 5/5 passing
  (install + idempotent + stale reject + unmanaged reject + global + project)
• `cargo test test_validate_init_target_selection` — OMP flag
  conflicts rejected as expected
• `bun --eval 'await import(".../hooks/omp/rtk.ts")'` loads
  cleanly (self-contained, no `@oh-my-pi/pi-coding-agent` runtime
  dependency that was causing `pi_natives` TLS failures on WSL2)
• Live OMP session: `git status`, `ls src`, `ls -la src`
  observed to be transparently rewritten to RTK's compact output

🤖 Generated with Claude Code

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[makoMakoGo]

I have used this pr for about 2 days on my omp.