/feedback opens the Kigi GitHub issues page

Feedback about an unofficial community build belongs on its own issue
tracker, not Moonshot's feedback endpoint — and this mirrors the
official kimi-cli, whose /feedback opens its repo's issues page
(ISSUE_URL in ui/shell/slash.py). /feedback now returns
Action::OpenUrl(https://github.com/ZacharyZhang-NY/Kigi-CLI/issues),
the same battle-tested browser path /docs web uses.

The now-dead TUI text-feedback pipeline is excised: PromptInputMode::
Feedback (~ prefix composer mode), Action::{EnterFeedbackMode,
SendFeedback}, Effect::SendFeedback (the kigi/feedback ACP POST),
TaskResult::{FeedbackComplete,FeedbackFailed}, and their dispatchers.
The shell-side kigi/feedback ACP extension stays: it is protocol
surface for editor embeddings, OAuth-gated, and shared with kigi/btw.

Gates: workspace check/clippy --all-targets 0/0, fmt, kigi-tui lib
6621 passed / 0 failed.

crates/codegen/kigi-tui/src/app/dispatch/task_result.rs

@@ -663,17 +663,6 @@ pub(super) fn dispatch_task_result(result: TaskResult, app: &mut AppView) -> Vec
             }
             vec![]
         }
-        TaskResult::FeedbackComplete { .. } => vec![],
-        TaskResult::FeedbackFailed { agent_id, error } => {
-            if let Some(agent) = app.agents.get_mut(&agent_id) {
-                agent
-                    .scrollback
-                    .push_block(crate::scrollback::block::RenderBlock::system(format!(
-                        "Couldn't send feedback: {error}"
-                    )));
-            }
-            vec![]
-        }
         TaskResult::MemoryNoteSaved { agent_id, result } => {
             handle_memory_note_saved(app, agent_id, result)
         }