## Summary

Implements user-defined slash commands for the agent panel. Users can
create markdown files in `~/.config/zed/commands/` (user-wide) or
`.zed/commands/` (project-specific) that expand into templated text when
invoked via `/command_name` in the chat interface.

## Features

- **File-based commands**: Create `.md` files that become slash commands
- **Template expansion**: Use `$1`, `$2`, etc. for positional arguments,
or `$ARGUMENTS` for all args
- **Namespacing**: Subdirectories create namespaced commands (e.g.,
`frontend/component.md` → `/frontend:component`)
- **Project & user scopes**: Project commands in `.zed/commands/`, user
commands in config dir
- **Claude compatibility**: Can symlink `~/.claude/commands/` for
compatibility
- **Caching**: Commands are cached and watched for file changes
- **Error handling**: Graceful degradation with error display in UI

## Feature Flag

Behind `user-slash-commands` feature flag.

## Testing

52 unit/integration tests covering parsing, validation, expansion, file
loading, symlinks, and error handling.

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Release Notes:

- N/A (behind feature flag)

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Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>