feat(version): extension reports its version; doctor shows all-component coherence

The upgrade story spanned four parts (CLI, daemon, extension, skill) with no
single view and — worst — the extension was a total black box: nothing reported
which build was live, so a user could sit on a stale extension with zero signal.

- ext (0.4.7): on connect the extension sends a `hello` with
  chrome.runtime.getManifest().version; the native-messaging host records it to a
  `relay-ext-version` sidecar (next to relay-cdp-url, removed on exit).
- build.rs embeds the shipped extension version (AB_CONNECT_VERSION, read from the
  ext manifest at compile time) so the CLI knows what extension it expects.
- `chrome-use doctor` gains a Versions section: CLI (vs the cached latest from the
  background update check), extension (connected version vs the bundled expected —
  warns + tells you to reload it in Chrome if behind), and skill (bundled, version-
  locked; `skills add` copies may be stale). Daemon coherence was already covered.

So 'which of the four parts is on what version, and what needs upgrading' is now
one command. Verified: doctor warns on a simulated old extension and passes on a
current one; gracefully shows 'not connected / predates reporting' when the host
hasn't learned a version yet.

extensions/ab-connect/background.js

@@ -108,6 +108,12 @@ function connectHost() {
     // reconnect. Keep chrome.debugger attached so reconnect is cheap.
     for (const tabId of tabs.keys()) setBadge(tabId, 'connecting')
   })
+  // Report our version so the host can tell the CLI/`doctor` which extension
+  // build is live (otherwise the extension version is a black box — the user
+  // can't tell they're on an old one). Best-effort; ignored by older hosts.
+  try {
+    postToHost({ method: 'hello', version: chrome.runtime.getManifest().version })
+  } catch {}
   // Tell the daemon about everything we already have attached, then attach
   // anything new.
   reannounceAttachedTabs()