fix(tui): rewire theme watcher lost in tab-view rewrite

[Sayt-0]

Fixes #3501

Editing ~/.cagent/themes/*.yaml while the TUI runs did nothing until restart, despite the docs promising auto-reload on save.

Root cause

The tab-view rewrite (0069cad) rewrote pkg/tui/tui.go and kept only the ThemeFileChangedMsg handler: nothing constructed styles.ThemeWatcher or emitted the message anymore (NewThemeWatcher had no non-test caller). Confirmed by reproduction: an e2e test editing the active theme file times out waiting for the hot-reload on main, and passes with this change.

Fix

runTUIWrapped / tuitest
  SetProgram(p)
    creates ThemeWatcher, callback: p.Send(ThemeFileChangedMsg)
    watchCurrentTheme()                          initial Watch

ChangeThemeMsg        > handleChangeTheme        > ThemeChangedMsg \
ThemePreviewMsg / ThemeCancelPreviewMsg ..........................> applyThemeChanged()
ThemeFileChangedMsg   > handleThemeFileChanged   > ThemeChangedMsg /   watchCurrentTheme() re-Watch

cleanupManagedResources()
  themeWatcher.Stop()                            both exit paths, once-guarded

Issue acceptance criterion	Handled by	Status
Editing the active custom theme updates the running TUI within ~1s (debounce), incl. atomic-save editors	SetProgram wiring; atomic saves already handled by the watcher (directory watching, rename/basename matching)	yes, e2e TestTheme_HotReload
Switching themes re-targets the watcher	re-Watch in applyThemeChanged, the single funnel for picker selection, preview, cancel and hot reload	yes, unit test
Exiting the TUI stops the watcher (no goroutine/fd leak)	Stop() in cleanupManagedResources, reached from both normal exit and external cancellation	yes, unit test
Regression test so the next TUI refactor cannot silently drop the wiring	e2e/tui/theme_test.go (red/green verified against main) plus pkg/tui/theme_watcher_wiring_test.go	yes

Notes

• themeWatcher sits behind a small themeFileWatcher interface so unit tests can record Watch/Stop calls.
• Nil watcher (embedders or tests that never call SetProgram) is a no-op; built-in-only refs already no-op inside the watcher.
• p.Send after program termination is a documented no-op in bubbletea v2, so a late debounce callback cannot block or crash.
• The e2e test re-edits the file on each poll: the watcher arms asynchronously (ThemeChangedMsg is sequenced after the change notification), so a single write could race fsnotify attachment. The 700ms poll interval exceeds the 500ms debounce so rewrites cannot starve the timer.
• docs/features/tui/index.md needs no change: its hot-reload claim is accurate again.

Validation

• go build ./...
• go test -race ./e2e/tui/ ./pkg/tui/ ./pkg/tui/styles/ ./cmd/root/
• golangci-lint run pkg/tui/... e2e/tui/... reports 0 issues