vscode: 'Open Dev URL' row in the Workspace view (opens default browser) #780

Context

Codev: Run Dev Server (workspace + worktree variants) spawns the dev
PTY inside VSCode, but the user still has to find the URL and open it
themselves. A one-click "open the running app" affordance, surfaced
right in the sidebar, closes that gap.

Proposal

Strictly manual — no auto-open, no settings, no lifecycle tracking.

1. New optional config in .codev/config.json, sibling to
   worktree.devCommand:

   "worktree": {
     "devCommand": "pnpm dev",
     "devUrl": "http://localhost:3000"
   }

2. New workspace-view row: Open Dev URL, rendered immediately
   below the existing Start Dev Server / Stop Dev Server row.
   Visible whenever worktree.devUrl is configured — independent of
   whether the dev PTY is currently running (the row click should work
   even if the server isn't up; the browser shows whatever the server
   is or isn't serving, which is the user's call).
3. New command: Codev: Open Dev URL. Backs the row click;
   palette-visible for users who prefer keyboard.
4. Click behavior: vscode.env.openExternal(vscode.Uri.parse(devUrl))
   — uses the user's default browser (or whatever they've mapped via
   workbench.externalUriOpeners).

Why default browser, not VSCode's Simple Browser

A real browser is the honest fit for a dev preview:

• DevTools / Console / Network / breakpoints / React DevTools — the
  dev loop is overwhelmingly DevTools-bound, and Simple Browser has
  none of them.
• Real cookie jar. Simple Browser loads the target in an iframe inside
  vscode-webview://, making it a third-party context; cross-site
  OAuth / SSO can break because Chromium third-party cookie rules
  block writes mid-flow.
• No framing restrictions — sites with X-Frame-Options: DENY /
  strict frame-ancestors (auth providers, dashboards) render fine
  in a real browser, blank in Simple Browser.

Users who specifically want embedded preview can still invoke Simple
Browser themselves via Command Palette → "Simple Browser: Show".

Why no auto-open

Considered and dropped: there's no API to detect whether the user
already has the URL open in their browser, and vscode.env.openExternal
always opens a new tab. Any auto-firing rule either piles up tabs (if
fired on every event) or guesses wrong (no signal to base it on).
Making the action manual sidesteps the whole class of problem and is
arguably what a developer wants anyway — explicit, predictable, no
surprises during dev-server restarts. The row is one click away, which
is the same cost as a clever auto-open without any of the footguns.

Acceptance

• worktree.devUrl recognized in .codev/config.json (validation
  + doc update).
• Codev: Open Dev URL command registered (palette-visible).
• Workspace-view row "Open Dev URL" appears directly below the
  Start/Stop Dev Server row whenever worktree.devUrl is set.
  Hidden when it isn't.
• Click → vscode.env.openExternal(devUrl) opens in the default
  browser; works regardless of whether the dev PTY is running.
• README updated with the new config block + the workspace-view
  row.

Out of scope

• Auto-open on dev-server start (explicitly rejected — see rationale
  above).
• Embedded Simple Browser preview (deferred; not relevant to a
  DevTools-bound dev loop).
• Multi-URL dashboards (only the configured devUrl).
• Browser tab activation / "switch to existing tab" (not possible via
  standard APIs; manual click is explicit by construction).