## Problem

Omarchy already has a single launch path for the selected coding agent through `omarchy-agent`. It is used by the keyboard shortcut, `a`, and `omarchy agent prompt`.

External controllers can currently integrate through individual harness hooks, but there is no generic way to inspect a prompted request before Omarchy launches the fallback agent.

This makes it difficult to build optional routers without adding each router to Omarchy's default-agent list.

Related work:

- [Custom coding agents discussion #10090](https://github.com/omacom/omarchy/discussions/10090)
- [Extensible agent harnesses #10171](https://github.com/omacom/omarchy/pull/10171)
- [User usage collector discovery #9602](https://github.com/omacom/omarchy/issues/9602)
- [Collector extension PR #12353](https://github.com/omacom/omarchy/pull/12353)

## Proposed extension

After `omarchy-agent` resolves the default agent and parses an optional prompt, check for an executable user-owned dispatcher:

```text
~/.config/omarchy/agents/request-dispatch
```

The launcher would call it with argv such as:

```text
--fallback-agent claude
--cwd /path/to/project
--prompt "Review this project"
```

The contract can stay small:

- Exit `0`: the dispatcher handled the request. Omarchy exits without launching the fallback.
- Any other exit status: continue with the existing Omarchy launch path.
- No dispatcher, or no prompt: preserve current behavior.
- Pass values as argv. Do not evaluate a shell command string.
- Do not add router-specific logic to Omarchy.
- Do not add the dispatcher to the default-agent picker.

A controller such as [Sabi](https://github.com/vizuh/sabi) could use this hook to inspect the repository, active sessions and available harnesses. If it can prove that a request was accepted or started elsewhere, it exits successfully. If the controller is unavailable or uncertain, it returns nonzero and Omarchy launches the configured default agent as usual.

This keeps Omarchy responsible for agent installation, command flags, terminal behavior and fallback handling. The external controller owns routing policy and execution receipts.

## Acceptance criteria

1. With no dispatcher installed, existing tests and behavior remain unchanged.
2. A promptless `omarchy agent` launch remains unchanged.
3. A dispatcher can handle a prompt without starting a second agent.
4. A dispatcher that declines falls through to the current default-agent case.
5. Arguments containing spaces and shell metacharacters are passed safely.
6. The hook is documented as an opt-in user extension.

This is deliberately narrower than adding Sabi as another default agent. Sabi can remain an external routing layer, while Omarchy gets a reusable extension point.