Excise the Changelog feature

Kigi never publishes CDN changelogs, so the entire inherited feature
was dead weight: the welcome-menu Changelog row, the hero-box info
slot (bullets + clickable CTA), /release-notes with its /changelog
alias, and the ChangelogManager CDN-fetch/disk-cache pipeline
(Effect::FetchChangelog, TaskResult::ChangelogFetched, startup and
post-login fetch kickoffs, AppView cache fields, mouse hover/click
handling). Welcome menu is now [Import] / New worktree / Resume
session / Quit; the hero box keeps title + version + subtitle and its
layout math simplifies to 3 + menu rows (verified equivalent by the
surviving boundary tests). Action::ShowReleaseNotes and the DocViewer
modal stay — /docs uses them. builtin.rs keeps deleting stale
CHANGELOG.{json,md} caches written by kigi ≤ 0.1.0.

kigi-shell-base drops its reqwest 'blocking' feature (only the deleted
module used it). -1206 lines net.

Gates: fmt clean, workspace check/clippy --all-targets 0/0, kigi-tui
lib 6609 / shell-base 56 / shell util:: 258 all passing, welcome pty
e2e (3 tests incl. braille logo) passing.

crates/codegen/kigi-tui/src/app/effects/mod.rs

@@ -1712,27 +1712,6 @@ pub(crate) fn execute(
                     TaskResult::PromptImagePreviewPrepared
                 });
         }
-        Effect::FetchChangelog => {
-            tasks
-                .spawn(async move {
-                    let changelog = tokio::task::spawn_blocking(|| {
-                            kigi_shell::util::changelog::ChangelogManager::new()
-                                .fetch()
-                        })
-                        .await
-                        .unwrap_or_else(|e| {
-                            tracing::warn!(error = % e, "changelog fetch task failed");
-                            kigi_shell::util::changelog::Changelog {
-                                markdown: None,
-                                entries: None,
-                            }
-                        });
-                    TaskResult::ChangelogFetched {
-                        markdown: changelog.markdown,
-                        entries: changelog.entries.unwrap_or_default(),
-                    }
-                });
-        }
         Effect::PersistMemoryFullscreen { fullscreen } => {
             persist_hint(
                 tasks,