Feature: user-defined slash-command workflow macros

## Problem

Buzz can pass slash commands through to one ACP agent, and workflows can fan out messages, but users cannot define a stable bare command that invokes a reusable workflow. A command such as `/new-task Build feature XYZ` should be able to dispatch deterministic, channel-scoped steps without asking an LLM to rediscover the routing recipe each time.

This is distinct from agent-owned commands. `@Agent /review ...` should continue to pass through to that agent runtime.

## Proposed contract

Add a workflow trigger:

```yaml
trigger:
on: slash_command
command: new-task
```

The command name omits `/`, uses a portable lowercase alphanumeric/hyphen shape, and matches only an exact bare leading token. The remainder is exposed as `{{trigger.args}}` and the name as `{{trigger.command}}`.

```yaml
name: New task
trigger:
on: slash_command
command: new-task
steps:
- id: plan
action: send_message
text: "@Planner /plan {{trigger.args}}"
reply_in_thread: true
- id: review
action: send_message
text: "@Reviewer /review {{trigger.args}}"
reply_in_thread: true
```

The existing `buzz workflows create/update/delete` CLI then gives managed agents a generic authoring path: a user can ask an agent to create or change a macro, and the agent can translate the request into workflow YAML after checking the channel for name collisions.

## Routing and safety

- Bare `/name ...` belongs to a channel workflow macro.
- Mention-prefixed `@Agent /name ...` remains ACP pass-through.
- Workflow loop suppression remains unchanged.
- Macro creation does not imply invocation; agents should save and report the definition without running it unless separately requested.
- Secrets must not be stored in macro definitions or arguments.
- Duplicate enabled names should be surfaced as a conflict, not silently selected.

## Scope

In scope: workflow schema/runtime matching, argument templates, Desktop workflow authoring, docs, and the bundled Buzz CLI skill guidance for agent-authored macros.

Out of scope: composer command autocomplete (#2528, #5700, #6567), a new Nostr kind, global macros, and a new agent-invocation action (#3114/#6060).

Agent fan-out through `send_message` also depends on resolving the existing trusted workflow wake defect (#3858); this feature should not weaken ACP author policy to work around it.

## Acceptance

- `/new-task Build feature XYZ` fires a `new-task` workflow and sets `trigger.args` to `Build feature XYZ`.
- `/new-task-extra` does not match `new-task`.
- `@Hermes /new-task ...` does not match the workflow.
- Names are validated consistently in Rust and Desktop.
- Existing workflow steps and `reply_in_thread` work unchanged.
- A managed agent can discover, create, update, and remove macros through the existing CLI.