/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/views/prompt_widget/mod.rs

@@ -174,7 +174,7 @@ pub struct PromptStyle {
     pub prefix_override: Option<(&'static str, ratatui::style::Color)>,
     /// Override the placeholder text shown when the textarea is empty.
     /// When `Some(text)`, uses this instead of the default `"Build anything"`.
-    /// Used for feedback mode (`"Type your feedback..."`).
+    /// Used for remember mode (`"Save a memory note..."`).
     pub placeholder_override: Option<&'static str>,
     /// Compact mode (currently unused for info_block sizing).
     pub compact: bool,