refactor(demo): drive recording through real cli.py instead of a harness

The previous demo harness called the backend Python API directly,
which didn't match what a user typing `./cli.py start <agent>` would
actually see. The recording now goes through the real CLI surface:

- claude-bottle.demo.json + scripts/demo-setup.sh stage a demo
  manifest (one bottle, FAKE_TOKEN env, one unreachable git upstream)
  alongside a dummy SSH identity at ~/.cache/claude-bottle-demo/.
- docs/demo.tape types `./cli.py start demo`, answers the y/N
  preflight, and runs four bash probes via claude's `!` prefix
  (curl x3 + git push), so the recording shows real preflight output
  and real probe results.
- scripts/demo.sh wraps setup -> cli.py -> teardown for human use;
  scripts/demo-record.sh does the same around `vhs docs/demo.tape`.
- .gitignore picks up claude-bottle.json so a user's local manifest
  doesn't get tracked alongside .example / .demo siblings.

scripts/demo_harness.py is removed -- its behavior is fully replaced
by the cli.py + `!` flow.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude-bottle.demo.json

@@ -0,0 +1,25 @@
+{
+  "bottles": {
+    "demo": {
+      "env": {
+        "FAKE_TOKEN": "ghp_aB3cD4eF5gH6iJ7kL8mN9oP0qR1sT2uV3wX4yZ"
+      },
+      "git": [
+        {
+          "Name": "foo",
+          "Upstream": "ssh://git@upstream.invalid/path.git",
+          "IdentityFile": "~/.cache/claude-bottle-demo/fake-key",
+          "KnownHostKey": "ssh-ed25519 AAAAEXAMPLE"
+        }
+      ]
+    }
+  },
+
+  "agents": {
+    "demo": {
+      "bottle": "demo",
+      "skills": [],
+      "prompt": ""
+    }
+  }
+}