Ambient Context v1.0.1

The app now updates itself. A 1.0.0 install has no updater and must be
replaced by hand with this release; from 1.0.1 on, a new version arrives
as a notification and a menu bar item.

Automatic updates

• Background check thirty seconds after launch and every six hours,
  on its own thread so it runs whether or not a folder or agent is set
  up. Offline or a GitHub error is logged and never shown.
• One notification per version per launch: "Version X is available.
  Update from the menu bar."
• Menu bar: a standing Check for Updates… item under About…. While
  a version is held it reads Update to X…; clicking it downloads,
  verifies the signature, installs, flushes the open capture block the way
  Quit does, and relaunches. During the install it reads Installing X…
  and is disabled. With nothing held it runs the check and reports the
  outcome.
• About window: a Check for updates button that runs the same
  check with its dialogs.
• Setting: Check for updates automatically, on by default, under
  Application; also check_updates over MCP set_config. Off leaves the
  manual paths only.
• The privacy document's network table says when the check runs and what
  it sends.

Release and docs

• Disk image renamed. Releases carry Ambient-Context-<version>.dmg
  and a fixed-name Ambient-Context.dmg, so a direct download link stays
  valid from one release to the next.
• README rewritten around why the record exists, with product
  screenshots, the 1.0.0 demo, and a download button above the fold.