[FEATURE]: /approve — toggle permission approval mode at runtime (on/off, per-session)

Requested feature

Add an /approve on|off slash command (inspired by Claude Code's /approve on/off) to toggle manual step-by-step permission approval at runtime, optionally per session.

Why

Permissions mode is currently fixed at startup: packages/tui/src/context/permission.tsx reads args.auto and exposes an in-memory toggle(), and the CLI only supports --auto / -y / --dangerously-skip-permissions flags (packages/opencode/src/cli/cmd/run.ts). There is no interactive command to switch between normal and auto approval mid-session.

Proposed behavior

• /approve on sets approval mode to auto; /approve off restores manual confirmation.
• Toggle is persisted per-session (surface in PermissionV1.Ruleset / session config) and honored by the existing permission evaluation path (packages/opencode/src/permission).

Benefits

• Go hands-free when safe, re-enable guards when tasks become risky, without restarting.
• Mirrors Claude's /approve on|off and the existing /yolo-style flag.