refactor(command): remove old Command enum and dispatch system

Remove the old JSON-based command infrastructure:

- Delete Command enum and CommandResult from types.rs
- Remove QuitCmd and InitBuffer command implementations
- Delete entire dispatch.rs file that handled command execution
- Remove Command type exports from mod.rs

The old system used a monolithic Command enum with serde serialization.
The new trait-based system is more flexible and doesn't require JSON
serialization for command execution.

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

src/command/cmd.rs

@@ -238,23 +238,6 @@ impl Cmd for EnterMode {
     }
 }
 
-#[derive(Debug)]
-pub struct QuitCmd;
-impl Cmd for QuitCmd {
-    fn name(&self) -> &str {
-        "quit"
-    }
-    fn execute(&self, ctx: &CmdContext) -> Vec<Box<dyn Effect>> {
-        if ctx.dirty {
-            vec![effect::set_status(
-                "Unsaved changes! Use :wq to save+quit or :q! to force quit",
-            )]
-        } else {
-            vec![effect::change_mode(AppMode::Quit)]
-        }
-    }
-}
-
 #[derive(Debug)]
 pub struct ForceQuit;
 impl Cmd for ForceQuit {
@@ -1364,28 +1347,6 @@ impl Cmd for PopChar {
     }
 }
 
-/// Initialize a named buffer (set it to a value) and change mode.
-#[derive(Debug)]
-pub struct InitBuffer {
-    pub buffer: String,
-    pub value: String,
-    pub mode: AppMode,
-}
-impl Cmd for InitBuffer {
-    fn name(&self) -> &str {
-        "init-buffer"
-    }
-    fn execute(&self, _ctx: &CmdContext) -> Vec<Box<dyn Effect>> {
-        vec![
-            Box::new(effect::SetBuffer {
-                name: self.buffer.clone(),
-                value: self.value.clone(),
-            }),
-            effect::change_mode(self.mode.clone()),
-        ]
-    }
-}
-
 // ── Commit commands (mode-specific buffer consumers) ────────────────────────
 
 /// Commit a cell edit: parse buffer, set cell, advance cursor, return to Normal.