Expand bot command surface

[crankshift]

Summary

Expand the Telegram bot command surface for session, project, OpenCode server, skills, MCP, filesystem, and scheduled-task workflows.

Context

The roadmap includes commands beyond the currently shipped surface. Command definitions should remain centralized, Telegram UX should stay in the adapter, and messenger-neutral behavior should stay in core services.

Acceptance Criteria

• /status reports server health, current session, and model info.
• /new creates a new session.
• /abort aborts the current task.
• /detach detaches from the current session without stopping it.
• /sessions browses and switches between recent sessions.
• /open adds a project by browsing directories.
• /ls lists directory contents and supports opening or downloading entries.
• /rename renames the current session.
• /commands browses and runs custom commands.
• /skills browses and runs OpenCode skills.
• /mcps browses and toggles MCP servers.
• /task creates a scheduled task.
• /tasklist browses and deletes scheduled tasks.
• /opencode_start starts the local OpenCode server on the bot machine.
• /opencode_stop stops the local OpenCode server on the bot machine.
• /help reflects available commands from the centralized command definitions.

Notes

/voice is already implemented for Telegram voice mode and should not be treated as unfinished scope unless follow-up behavior is explicitly identified. This large item may need to be split into smaller implementation tasks before coding.