feat(desktop): open t3code://threads links in the desktop app

[bogorman]

Problem

The mobile app opens t3code://threads/<environmentId>/<threadId>, and agent awareness emits that shape, but the
desktop app only raises its window (pingdotgg#9745). Other tools can only link to a thread through the local
server's web address, which opens a browser tab instead of the app.

Change

Thread links ride the existing web-link path instead of a new one:

• Main process: the open-url handler recognises thread links. DesktopWebLinks keeps the newest one until the
  renderer says it is listening (cold start, reload, crash), then sends it and reveals the window.
• No window open (macOS): when a link arrives with no window open, one opens the way a Dock click does
  (DesktopWindow.activate), and the link waits for it.
• Renderer: DesktopWebLinkCoordinator navigates to /$environmentId/$threadId.
• Index route: it no longer starts a draft over a thread link that already navigated away (this fix is from feat(desktop): open thread deep links via t3code:// pingdotgg/t3code#8246).
• Strict parsing: the scheme, the threads host, a UUID environment id and one URI-encoded thread segment.
  Anything else is ignored, so the scheme can only open a thread. Sign-in links are untouched.
• Docs: docs/user/thread-links.md.

105 lines of app code plus tests. This is a smaller take on pingdotgg#8246, which covers the same links with its
own subscribe/acknowledge handshake. It reuses the web-link delivery the app already has, and it also opens a window
when none is open, which pingdotgg#8246 doesn't.

Not covered: Windows and Linux (links arrive as process arguments there), and the t3code://app/... alias.

Testing

• DesktopWebLinks.test.ts: parsing (valid, encoded, wrong scheme or host, extra segments, query, fragment, bad
  encoding), and holding the newest link until the renderer listens, delivered once.
• DesktopClerk.test.ts: a thread link is handled and opens a window; non-thread links are left alone.
• Desktop suite: 1,447 tests pass. Web suite: 6,599 tests pass. Type checks and format are clean.
• Manual check on a packaged macOS build is still to do: cold start, app running, window closed, minimised.

🤖 Generated with Claude Code

Closes #4