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/dispatch/auth.rs

@@ -387,9 +387,6 @@ pub(super) fn handle_auth_complete(
         // status only; shell auto-syncs post-auth
         let mut effects = dispatch(Action::RequestBundleStatus, app);
 
-        // Fetch changelog (mirrors startup path for interactive login).
-        effects.push(Effect::FetchChangelog);
-
         // Replay deferred session startup once BOTH gates are open. Auth
         // is now Done, so `session_startup_allowed()` here means "is trust
         // also resolved?" -- if trust is still Pending its question renders