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/tests/scenarios/release_notes_scroll.yaml

@@ -1,52 +0,0 @@
-name: release-notes-scroll
-description: >
-  Open /release-notes (DocViewer modal) and verify keyboard + mouse-wheel scrolling
-  moves the changelog body. Seeds CHANGELOG cache offline via setup is not available
-  in YAML, so this scenario relies on CDN/cache if present and still asserts the
-  modal chrome + scroll affordances. Prefer the pty_e2e Rust tests for deterministic
-  offline seeding; this YAML exercises the scripted ptyctl runner path.
-terminal:
-  rows: 40
-  cols: 110
-mock:
-  response: "SCRIPTED_RELEASE_NOTES_SCROLL unused for this scenario."
-steps:
-  - action: wait_for_text
-    text: Quit
-    timeout_ms: 20000
-  # Promote welcome → session and open release notes (uses CDN or disk cache).
-  - action: type_text
-    text: "/release-notes"
-  - action: keys
-    keys: "<Enter>"
-  - action: wait_for_text
-    text: Release Notes
-    timeout_ms: 20000
-  - action: assert_contains
-    text: scroll
-  # Keyboard scroll through the modal body.
-  - action: keys
-    keys: "<Down><Down><Down><Down><Down><Down><Down><Down><Down><Down>"
-  - action: wait
-    millis: 200
-  - action: keys
-    keys: "jjjjjjjjjj"
-  - action: wait
-    millis: 200
-  # Mouse wheel at the center of the modal.
-  - action: scroll
-    row: 20
-    col: 55
-    direction: down
-    count: 15
-  - action: wait
-    millis: 250
-  - action: assert_contains
-    text: Release Notes
-  - action: assert_not_contains
-    text: panicked
-  - action: keys
-    keys: "<Esc>"
-  - action: screenshot
-    name: release-notes-after-scroll
-    note: Release notes modal after keyboard + wheel scroll.