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/tests/mod.rs

@@ -85,8 +85,6 @@ fn test_app() -> AppView {
         pending_notification_escapes: None,
         deferred_notification: None,
         tracing_rx: None,
-        changelog_markdown: None,
-        changelog_bullets: Vec::new(),
         tips: Vec::new(),
         tip: None,
         cli_model_override: None,
@@ -143,7 +141,6 @@ fn test_app() -> AppView {
         welcome_tip_typing_dismissed: false,
         welcome_menu_index: None,
         welcome_menu_rects: Vec::new(),
-        welcome_show_changelog_action: false,
         welcome_import_banner_rect: None,
         last_mouse_pos: None,
         last_scroll_pos: None,
@@ -151,9 +148,7 @@ fn test_app() -> AppView {
         welcome_prompt_rect: None,
         welcome_auth_url_rect: None,
         welcome_on_auth_url: false,
-        welcome_on_changelog_cta: false,
         welcome_auth_fallback_rect: None,
-        welcome_changelog_cta_rect: None,
         auth_show_raw_url: false,
         auth_mouse_disabled: false,
         session_picker_entries: None,