Add slash command support to Crush editor

[heynemann]

Idea from #523

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This commit implements comprehensive slash command support similar to Claude Code, allowing users to create custom commands that can be invoked with \command-name syntax.

Key Features:

• Slash command syntax using backslash prefix (\command-name) to avoid conflicts
• Custom commands loaded from multiple locations with precedence:
  • Project: .crush/commands/
  • User home: ~/.crush/commands/
  • XDG config: $XDG_CONFIG_HOME/crush/commands/
• YAML frontmatter support for command metadata:
  • description: Command description shown in help and completions
  • argument-hint: Hints for expected arguments (e.g., [arg1] [arg2])
  • allowed-tools: List of Crush tools allowed for this command
• Argument parsing and substitution:
  • Supports $ARGUMENTS for all arguments
  • Supports positional arguments $1, $2, etc.
  • Automatic argument detection from argument-hint or content placeholders
  • Appends arguments to prompt when not all are referenced in content
• File reference support using @filename syntax
• Namespacing using directory structure (e.g., frontend:review-pr)
• Built-in \help command listing all available commands
• Fuzzy command completion with backslash trigger
• Command reload via Ctrl+P "Reload Commands"
• Comprehensive error handling and validation
• Full test coverage with unit and integration tests

Implementation Details:

• Command registry with loading, lookup, and reload functionality
• Command executor integrating with agent coordinator
• Tool filtering based on allowed-tools frontmatter
• Markdown-formatted help output with purple command styling
• Editor integration for command detection and execution
• Session auto-creation when executing commands without active session

All commands are sorted lexicographically and grouped by namespace in help output. Commands can be reloaded without restarting the application.

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

[charmcli]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[heynemann]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.

[meowgorithm]

Hi and thanks for the contribution! While we appreciate the effort here, we're going to close this on in favor of #1377 as it's simpler and more in-line with our current thinking.

Please note that with PRs for features we request that you chat with the core team (typically in a GitHub discussion or on Slack or Discord) prior to submitting the PR in order to make sure the feature aligns with the project direction.