v0.3.0

• Added the userland origin and a new @extension.dev/urls/userland entry
  point covering the public build viewer: userlandOrigin,
  userlandProjectPath, userlandUrl, userlandBuildUrl,
  userlandChannelUrl, userlandDialogUrl, plus the UserlandProjectPage
  tails and the reserved USERLAND_BUILD_TABS.
• userland is the one app whose production host is per-workspace
  (<workspace>.extension.dev), so Origins.userland holds the apex and the
  module derives the real origin. In local dev the workspace moves into the
  path instead, and the mode is derived from the base rather than passed, so a
  caller cannot get prod and dev backwards.
• This closes the last hand-rolled cross-app link: the share URL
  POST /api/cli/publish returns (what extension_publish hands an agent to
  pass to a human) was concatenated from a local constant and could drift from
  the routes userland actually serves.