Scope-aware input handler for the active surface handler list.

Runs after modal overlay handlers and before the mode FSM fallback. Resolves keys through the active keymap scope, handling scope-specific bindings, prefix sequences, and sub-state dispatch (search input, mention completion).

For the :editor scope, keys pass through to the mode FSM unless the agent side panel is visible, in which case panel-specific handling applies.

For the :agent scope, keys are resolved against the agent scope trie. Context-dependent sub-states (search input, mention completion, tool approval, diff review) are handled before trie lookup. The leader key (SPC) always passes through to the mode FSM for which-key integration.

For the :file_tree scope, tree-specific keys are handled via scope resolution and unmatched keys delegate to the mode FSM with the tree buffer swapped in as the active buffer (providing full vim navigation).