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-shell/src/builtin.rs

@@ -116,8 +116,8 @@ pub fn extract_bundled_files(kigi_home: &std::path::Path) {
 
     let _ = std::fs::create_dir_all(kigi_home);
 
-    // Clean up cached changelog files from previous version so
-    // /release-notes fetches fresh content for the new version.
+    // Clean up changelog caches written by the removed changelog feature
+    // (kigi <= 0.1.0 cached CDN release notes in the kigi home).
     for stale in &["CHANGELOG.json", "CHANGELOG.md"] {
         let _ = std::fs::remove_file(kigi_home.join(stale));
     }