Add runners for Aider, Codex CLI, and Gemini CLI

[that-github-user]

Summary

Add runner implementations for additional AI coding tools:

• Aider (aider)
• Codex CLI (codex)
• Gemini CLI (gemini)

Each runner implements the Runner interface in src/runners/base.ts.

Why

Multi-tool ensembles address the single-model diversity limitation. The runner interface is already pluggable — each tool just needs available() (check if CLI is installed) and run() (execute with prompt in worktree).

References

• Runner interface: src/runners/base.ts
• Registry: src/runners/registry.ts
• Existing Claude Code runner: src/runners/claude-code.ts