feat: install missing agents during setup

[SaladDay]

Follow-up to #242.

What changed

• Detect selected agents that are missing from PATH.
• Let users choose which missing agents to install.
• Use official installers or packages for all six supported agents.
• Run the official non-interactive Hermes stages. Do not run setup, gateway, or desktop stages.
• Verify each install before applying MiniMax configuration.
• Preserve existing agent settings.
• Align thinking controls with current MiniMax APIs and client behavior.
• Keep Hermes M3 thinking off by default because current Hermes sends a legacy payload when thinking is enabled.
• Keep dry-run and non-interactive setup configuration-only.
• Show clear network, proxy, timeout, permission, and interruption errors.
• Clean up setup locks and installer processes on SIGHUP, SIGINT, and SIGTERM.

Tests

• 503 tests passed.
• Type check passed.
• Lint passed with one existing warning.
• Production build passed.
• Installed and verified all six agents in clean environments.
• Installed current Hermes with all official non-interactive stages in an isolated home.
• Verified Codex, OpenCode, Pi, and Hermes request or config behavior with the real clients.
• Verified proxy, timeout, SIGHUP, SIGINT, SIGTERM, and network failure paths.
• Final blind review: clean.

Known upstream limit

Current Hermes does not emit the M3 adaptive thinking payload. The generated config uses none so first use works. This PR does not patch Hermes.