Recover from completion panics

[M09Ic]

Background

A panic during completion currently escapes the completion path and can bring down an interactive console. Completion is usually triggered while typing, so a single bad completer or transient command-tree state should degrade into a visible completion error instead of crashing the process.

Approach

• Add a deferred recover around Console.complete.
• Return a readline completion message containing the panic value.
• Reset the menu command state after a recovered panic so the next completion starts from a clean command tree.

Tests

• Added a regression test that forces a nil command-tree panic and verifies complete does not panic.
• Ran go test ./...