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/event_loop.rs

@@ -1136,12 +1136,6 @@ pub(crate) async fn run(
         if process_effects(effs, &mut tasks, &mut app, &progress_tx) {
             return Ok(make_run_result(&app));
         }
-        // Fetch changelog off the render path so the welcome screen
-        // can display bullets and /release-notes uses the cached result.
-        let effs = vec![super::actions::Effect::FetchChangelog];
-        if process_effects(effs, &mut tasks, &mut app, &progress_tx) {
-            return Ok(make_run_result(&app));
-        }
     }
 
     if !post_render_effects.is_empty()