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/command/keymap.rs

@@ -165,7 +165,7 @@ impl Keymap {
         let binding = self.lookup(key, mods)?;
         match binding {
             Binding::Cmd { name, args } => {
-                let cmd = registry.parse(name, args).ok()?;
+                let cmd = registry.interactive(name, args, ctx).ok()?;
                 Some(cmd.execute(ctx))
             }
             Binding::Prefix(sub) => Some(vec![Box::new(SetTransientKeymap(sub.clone()))]),
@@ -267,7 +267,7 @@ impl KeymapSet {
         normal.bind_args(KeyCode::Char('u'), ctrl, "scroll-rows", vec!["-5".into()]);
 
         // Cell operations
-        normal.bind(KeyCode::Char('x'), none, "clear-selected-cell");
+        normal.bind(KeyCode::Char('x'), none, "clear-cell");
         normal.bind(KeyCode::Char('p'), none, "paste");
 
         // View