Feature: --channels should support --resume to attach to existing sessions

Problem

The new --channels feature (research preview) creates a fresh session every time. There is no way to attach a channel (Telegram, Discord, etc.) to an existing session that already has rich conversation context.

This means if I have a long-running session where Claude deeply understands my codebase, project decisions, and ongoing work, I cannot connect a Discord/Telegram bot to that session to interact with it remotely. I have to start from scratch every time.

Proposed Solution

Allow --channels to work with --resume / --session-id:

claude --channels plugin:discord@claude-plugins-official --resume <session-id>

This would:

1. Resume the specified session with its full conversation context
2. Attach the channel MCP server to that resumed session
3. Allow external messages (Discord, Telegram, etc.) to interact with a session that already has deep context

Use Case

• I have a Claude Code session that has been working on my project for hours, with full understanding of architecture decisions, code changes, and ongoing tasks
• I want to step away from my desk and continue interacting with that same session via Discord/Telegram on my phone
• Currently --channels only creates new sessions, so all that context is lost

Why This Matters

The whole value of channels is remote interaction with Claude Code. But without session continuity, the remote client is limited to simple, context-free queries. The real power would be combining channels with session memory — talk to the same Claude that already knows your project deeply.

Current Workaround

None. --channels does not accept --resume or --session-id. The only option is keeping a terminal open with tmux, but that only works for the current session — you still can't attach channels to it after it's started.