Support tools for prompts

[RomneyDa]

Description

The problem: Slash commands are built with generators that output strings. Thus, they don't support tool calls.

This PR restructures slash commands to add tool support for prompts.

TL;DR: non-legacy slash commands (prompt blocks, mcp, etc.) are now rendered on the GUI side and included in messages as normal conversations

Details

• Adds a SlashCommandWithSource type (similar idea to RuleWithSource that adds source traceability and an isLegacy flag to slash commands, without breaking e.g. config.ts SlashCommand type
• Removes the run functions from non-legacy slash commands
• Updates editor content resolution on the GUI to render and inject slash command prompts on the spot. This includes
  • adding an mcp/getPrompt message that hits core and retrieves/stringifies an mcp prompt
  • moving prompt rendering logic to the GUI
• Moves all legacy slash commands e.g. /share, /commit, etc. to a new more isolated directory

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