Extension mode context detection

What Changed

Added ctx.mode to extension contexts allowing extensions to distinguish execution modes: 'tui' (Terminal UI), 'rpc' (RPC protocol), 'json' (JSON event stream), 'print' (print mode). Enables guards for terminal-only features like custom(), component factories, and direct TUI rendering.

Operator Implication

Extensions can now properly handle mode-specific behavior. Replaces runtime checks for hasUI with explicit mode detection, improving robustness.

Receipt

• Commit e56521e 'Add extension mode context' adds ctx.mode field allowing extensions to check 'tui', 'rpc', 'json', or 'print' execution modes