announcement: use opentui <a href> for clickable TUI URLs; revert Desktop markdown

The previous OSC 8 attempt did not work because opentui's <text> element
strips raw ANSI escape sequences before rendering. The proper opentui
mechanism is its built-in <a href="..."> JSX intrinsic (LinkProps,
emitted as a native OSC 8 hyperlink by opentui's renderer).

Switch the TUI startup announcement from api.ui.DialogAlert (string-only
message prop) to a custom JSX dialog via api.ui.dialog.replace. Bullets
are split into text/url segments and each URL renders as a real <a> with
underline styling. iTerm2, kitty, WezTerm, Alacritty, Ghostty, modern
macOS Terminal etc. now show the Discord link as Cmd-clickable; older
terminals fall back to plain styled text.

Also revert the Desktop markdown wrap. OpenCode Desktop renders user
messages through HighlightedText (packages/ui/src/components/message-part.tsx
~L1184), which is a plain <span> renderer — not Markdown, no URL
auto-linking. Wrapping URLs in [url](url) just showed literal brackets.
Document the upstream constraint and leave URLs as plain text so the
user can copy them. True Desktop clickability requires OpenCode to add
URL detection to HighlightedText.

Pi notify keeps OSC 8 (works since Pi passes ANSI escapes through to the
host terminal verbatim).