enhancement: implement /about command

[puremachinery]

Context

There is no way to see a concise summary of the current runtime state from inside the REPL. Users need to check --version, the config files, and their environment separately.

Proposed change

Add an /about slash command that displays a one-screen summary of the current session.

Suggested fields for the first version:

• Version string
• Active model name
• Auth method in use (OAuth / API key)
• Approval mode
• Config file paths (system, global, workspace)

Keep the first version intentionally small. More detailed diagnostics (for example account email or a full tool inventory) can be follow-up issues if needed.

Why this is a good first issue

• The data already exists in the runtime
• Follows the same slash-command pattern as /model, /chat, and /memory
• Mostly formatting and wiring, with no state mutation

Relevant files

• internal/ui/commands.go — slash command dispatch
• internal/version/version.go — version string
• internal/config/loader.go — config paths