Slash commands: registry, /cwd handler, and chat-input completion #5

Summary

Implement slash-commands per docs/design/slash-commands.md (not yet implemented).

Scope

• Add ISlashCommandHandler, SlashCommandContext, SlashCommandResult, ISlashCommandRegistry, and CompositeSlashCommandRegistry.
• Implement the /cwd handler.
• Wire chat-input interception + completion into AgentChatEditorControl.
• Add agent-session.cwd and AgentDefinition.workingDirectory schema support.

Acceptance

• Typing a slash command in chat input is intercepted and dispatched through the registry.
• /cwd reports/sets the working directory.
• Completion is offered for registered commands.
• Tests cover registry dispatch and the /cwd handler.

Related

• Shares agent-session.cwd / AgentDefinition.workingDirectory schema with the copilot working-directory work.