refactor(examples): fold the sandbox composition into acp-agent as its variant

sandbox-acp-agent's whole surface moves into examples/acp-agent following
the example's existing multi-variant shape (the code-mode/both-mode
overlays): sandbox.cordis.yml + its replay overlay, the four snapshot
scenarios (config-options / mode-switching / escalation-approved /
escalation-rejected) as a 'sandbox' header class over per-scenario
configPath — goldens byte-identical, zero re-recording — and
escalation.e2e.ts unchanged apart from the config path. One ACP example
remains; demo:sandbox-acp keeps its name and boots the variant. References
(both RFCs, group/package READMEs, the examples table and test map, the
e2e workflow comment) now point at the variant.

examples/acp-agent/tests/snapshots/config-options/input.json

@@ -0,0 +1,10 @@
+{
+  "steps": [
+    { "op": "initialize" },
+    { "op": "newSession" },
+    { "op": "setConfigOption", "configId": "sandbox-mode", "value": "workspace-write" },
+    { "op": "setConfigOption", "configId": "approval-policy", "value": "never" },
+    { "op": "setConfigOptionExpectError", "configId": "sandbox-mode", "value": "yolo" },
+    { "op": "setConfigOptionExpectError", "configId": "reasoning-effort", "value": "max" }
+  ]
+}