Support Copilot customization slash commands

[aeschli]

Copilot CLI provides slash commands for discovering and managing customizations. Copilot agent-host sessions in VS Code should support the same command names where the headless runtime exposes the required operations.

Initial VS Code integration: #337928

[aeschli]

Analysis

Agent-host sessions do not need to send recognized slash commands as ordinary model prompts. VS Code can use the Copilot SDK command API:

1. Call session.commands.list() to discover runtime commands.
2. Resolve the leading slash command and aliases from that catalog.
3. Call session.commands.invoke() for recognized non-skill commands.
4. Continue sending user-invocable skill commands through the normal skill path.

For /skills, the headless runtime already supports invocation. Bare /skills defaults to listing skills; supported headless subcommands are list, info, and reload.

Why only some customization commands work

The SDK exposes Copilot's Rust RUNTIME_COMMANDS registry, not the larger interactive CLI/TUI command registry.

Currently available through the SDK:

Command	Headless SDK support
/skills	list, info, reload
/plugin	list only
/mcp	list, show, enable, disable, reload
/init	Supported
User-invocable skills	Supported as slash commands

These remain CLI/TUI-only and are not returned by session.commands.list():

• /skills add, /skills remove, and the /skill alias
• /plugin install, uninstall, update, and marketplace
• /mcp add, edit, delete, auth, and search
• /instructions
• /agent
• /subagents and /agents
• /lsp
• /extensions

Supporting the complete set requires corresponding headless commands, aliases, and subcommand metadata in copilot-agent-runtime. Once advertised through the SDK contract, VS Code can discover and invoke them without implementing provider-specific copies of the operations.

Existing VS Code collisions

VS Code previously intercepted /agents, /skills, and /instructions for Copilot agent-host sessions and opened VS Code customization UI before the command reached Copilot. The initial integration removes those agent-host interceptions and exposes the runtime-advertised /skills, /plugin, and /mcp subcommands.

Initial implementation: #337928