feat: add init slash command

[boggedbrush]

Summary

This PR adds a new /init slash command to the composer so users can quickly create the harness instruction file for their current provider.

• Codex users get AGENTS.md
• Claude users get CLAUDE.md

If both harnesses are enabled and installed, /init also offers to create the counterpart file as a symlink so both harnesses can share the same instructions without duplication.

What’s included

• Added /init to slash command parsing and command menu
• Added harness-aware guide file creation based on the currently selected provider
• Added symlink prompt flow when both harnesses are available
• Added a safe workspace symlink RPC on the server
• Added tests for:
  • slash command parsing
  • init command helper logic
  • workspace symlink creation
  • websocket RPC routing

Why

Both Codex and Claude already support repo-level instruction files, so this makes setup faster and more discoverable directly inside Kodo Code. The symlink option also helps avoid maintaining duplicate instructions in repos that use both harnesses.

Verification

• bun fmt
• bun lint
• bun typecheck
• Targeted web and server Vitest coverage for the new /init flow and symlink support

Notes

• /init is idempotent for existing files and avoids creating duplicate guide files when the counterpart already exists.
• Symlink creation stays inside the workspace root and is handled through typed workspace RPCs rather than shelling out.