feat: add persistent browser server with auto-discovery

Add `glimpse serve` which starts geckodriver + Firefox and proxies
WebDriver requests through a Unix socket at
$XDG_RUNTIME_DIR/glimpse-<uid>.sock. All commands auto-discover the
socket and reuse the running browser session (~300ms vs ~2-3s per
command).

The proxy intercepts session create/delete to keep Firefox alive:
new session requests return the existing session ID, delete session
navigates to about:blank instead of closing Firefox.

- `glimpse serve` starts in foreground, logs to stderr
- `glimpse serve --stop` sends shutdown via socket
- `glimpse serve --status` prints JSON status
- SIGTERM/SIGINT do full cleanup (Firefox + geckodriver + socket)
- Second instance detected and rejected with exit code 1
- GLIMPSE_SOCKET_PATH env var for test isolation
- Three new smoke tests for serve lifecycle

AGENTS.md

@@ -42,6 +42,7 @@ Current `glimpse` subcommands:
 - `exec <url> --js=<code>` or `--script=<file>` - execute JavaScript and return the result
 - `screenshot <url> --output=<file>` - save a PNG screenshot
 - `search <query>` - search with a supported provider and output JSON results
+- `serve` - start a persistent browser server (geckodriver + Firefox) for faster repeat commands; auto-discovered via Unix socket, `--stop` to shut down, `--status` to check
 
 ## Runtime Requirements