feat(session): add /session command to browse and attach CLI sessions

[RoundTable02]

Summary

Add a new /session slash command that allows Discord users to browse, attach, and manage existing OpenCode CLI sessions directly from Discord threads — enabling seamless session resume across CLI and Discord.

Related Issue

Closes #25

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Changes Made

• New src/commands/session.ts — /session command with 4 subcommands:
  • list — Lists all sessions for the channel's bound project, showing session titles, mapping status, and last-used time
  • attach — Presents a select menu of active sessions (labeled by title) to attach to the current thread
  • detach — Detaches the session from the current thread and clears the SSE client
  • info — Shows detailed session info embed including title, ID, port, status, and timestamps
• Modified src/services/sessionManager.ts:
  • Added SessionInfo interface ({ id, title })
  • Updated listSessions() to return SessionInfo[] with titles from the OpenCode API
  • Added getSessionInfo() to fetch a single session's title and validate its existence
• Modified src/commands/index.ts — Registered the new session command

Testing

• I have tested this locally
• I have added/updated tests (if applicable)
• All existing tests pass (npm test)

Checklist

• My code follows the existing code style
• I have not included version bumps (maintainer handles versioning)
• I have updated documentation if needed
• This PR focuses on a single feature/fix

Additional Notes

• Session titles are sourced from the OpenCode API (GET /session returns title field per session). This matches the OpenCode CLI's /session TUI which displays titles for easy identification.
• The attach subcommand uses Discord's StringSelectMenu with a 120-second timeout. Select menu labels show session titles (max 25 chars), descriptions show session ID prefix and mapping info.
• After attaching, freshContext is automatically set to false so the resumed session preserves conversation history.
• After merging, run remote-opencode deploy to register the new slash command with Discord.