Fix rbw vault lock detection by using system rbw

The nix-bundled rbw (1.13.2) had a protocol mismatch with newer system
rbw-agent versions. The agent sends environment variables as byte arrays,
but rbw 1.13.2 expects base64-encoded strings, causing:

  rbw unlocked: failed to parse message '...': invalid type: sequence,
  expected base64 encoded os string

By not bundling rbw in the nix package, the script uses the system rbw
which matches the running rbw-agent and can communicate correctly.

Fixes gcr-tth

.gitignore

@@ -1,2 +1,5 @@
 .beads/
 __pycache__/
+.runtime/
+result
+state.json

flake.nix

@@ -12,7 +12,7 @@
         pkgs.python3Packages.selenium
         pkgs.chromedriver
         pkgs.chromium
-        pkgs.rbw
+        # Note: rbw intentionally NOT included - must use system rbw to match running rbw-agent
         pkgs.wl-clipboard
         pkgs.xclip
       ];