revert: drop opentui <a href> and OSC 8 from announcement; use plain text

Several attempts to make the Discord URL clickable failed in practice:

1. OSC 8 escape sequences in opentui <text>: stripped by the text
   sanitizer, never reached the terminal.
2. Custom JSX dialog with <a href> directly in <box>: 'Orphan text
   error' crash because <a> is span-like and requires a <text> parent.
3. <a href> inside <text> with nested <span> children: opentui treats
   each non-string child as a non-wrappable inline block, so bullets
   bled past the dialog border.
4. <a href> inside <text> with pure-string siblings: same bullet
   overflow behavior despite the simpler tree.

The pragmatic AFT pattern is plain string in DialogAlert. Modern
terminals (iTerm2, kitty, WezTerm, Ghostty, recent macOS Terminal)
auto-detect bare URLs and let users Cmd-click them. Older terminals
require manual copy. This is the most reliable surface across all
client configurations.

TUI side:
  • Restored api.ui.DialogAlert with plain string message.
  • Removed splitUrlSegments, AnnouncementDialog, BulletSegment type,
    SINGLE_BORDER_DIALOG sentinel, and the unused <a>/<span>/<u>/<b>
    JSX intrinsics — they're no longer referenced.
  • Documented the constraint inline so a future refactor doesn't
    re-attempt the clickable-URL path without understanding why it
    failed.

Pi side:
  • Removed the OSC 8 wrapper; ctx.ui.notify gets plain text.
  • Same documentation comment as TUI.