refactor(command): pre-resolve cell key and grid dimensions in CmdContext

Refactor command system to pre-resolve cell key and grid dimensions
in CmdContext, eliminating repeated GridLayout construction.

Key changes:
- Add cell_key, row_count, col_count to CmdContext
- Replace generic CmdRegistry::register with
  register/register_pure/register_nullary
- Cell commands (clear-cell, yank, paste) now take explicit CellKey
- Update keymap dispatch to use new interactive() method
- Special-case "search" buffer in SetBuffer effect
- Update tests to populate new context fields

This reduces code duplication and makes command execution more
efficient by computing layout once at context creation time.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/Qwen3.5-35B-A3B-GGUF:Q5_K_M)

src/ui/effect.rs

@@ -289,7 +289,12 @@ pub struct SetBuffer {
 }
 impl Effect for SetBuffer {
     fn apply(&self, app: &mut App) {
-        app.buffers.insert(self.name.clone(), self.value.clone());
+        // "search" is special — it writes to search_query for backward compat
+        if self.name == "search" {
+            app.search_query = self.value.clone();
+        } else {
+            app.buffers.insert(self.name.clone(), self.value.clone());
+        }
     }
 }