feat(app): add session message search - #45037

Search within the current session's message history -- open with
<leader>f or /find (alias /search), type a query, step between
matches with Enter/Shift+Enter, Esc to close.

Ported from upstream anomalyco#4714 (67 reactions/35
comments). Unusually, the relevant PRs (anomalyco#45037, anomalyco#48470) were still
open and unreviewed rather than bot-closed -- but neither actually
targets the TUI: both add Cmd/Ctrl+F search to the web/desktop
timeline instead. anomalyco#45037's approach (reusing the file viewer's search
by scanning the rendered page) can't carry over to a terminal UI.
anomalyco#48470's approach -- find matches directly from message data (text,
reasoning, tool inputs, error rows), then scroll to the hit -- doesn't
depend on a browser, so its matching logic (partSearchText,
findMatches) is ported directly into a new
packages/tui/src/util/session-find.ts. Its browser-only parts (DOM
highlighting, the web find-bar, file-viewer shortcut sharing) have no
TUI equivalent and were rewritten as a real terminal find-bar
component plus new match-navigation and scroll-target logic.

Four new overridable keybinds (session_find <leader>f,
session_find_next/previous/close). Matched parts get a highlighted
background, stronger on the current match; a collapsed "Thought"
block auto-opens while it holds the current match.

Known gaps: word-level highlighting only works in user messages
(assistant/reasoning/tool rows highlight as a whole block -- the
terminal renderer can't highlight part of a markdown block); tool
output isn't searched, only tool inputs, matching anomalyco#48470's own
choice; a match inside a hidden/collapsed region is counted but has
nothing on screen to jump to; only the current session is searched,
no cross-session search; keybinds docs and generated SDK types don't
list the new keys yet.

New packages/tui/test/util/session-find.test.ts (11 tests, adapted
from anomalyco#48470's own test file) and packages/tui/test/cli/tui/
session-find.test.tsx (2 tests rendering the real find bar). Verified
against this fork's current dev, including the expandable-pasted-
text-placeholders feature which touches two of the same files with
no conflict: bun typecheck clean; full packages/tui suite 228 pass.

Co-Authored-By: Claude Sonnet 5 <[email protected]>