feat: /sessions — list, resume, and switch between saved sessions

[VforVitorio]

What problem does this solve?

Users have no way to list, inspect, or resume previous sessions from within lmcode. After exiting lmcode chat, all conversation context is gone and there is no way to switch between ongoing tasks without starting from scratch.

Proposed solution

Add a /sessions slash command (and optionally lmcode session list/view) that lets the user list previous sessions, inspect them, and optionally resume one by injecting its summary as context.

/sessions              — list last N sessions (id, date, first message, turn count)
/sessions <id>         — show full session as bordered panels (like /history)
/sessions resume <id>  — /compact the current history, inject selected session summary

CLI equivalent (already stubbed):

lmcode session list
lmcode session view <id>

Depends on: #65 (session recorder must exist first)

Roadmap: v1.0

Alternatives considered

None considered.