docs(sandboy): sandbox_policy must fail closed, matching 007 loop-canvas

Keep the two sides of the contract consistent: the 007-side bullet framed
`sandbox_policy` as plain "forward-compatible", but 007/docs/loop-canvas.md
now requires it to fail closed (schema bump) since silent unknown-field
tolerance fails open on a security control.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VNgUjrvwqwtXqe8URbJaQN

Author: PhysShell

sandboy/README.md

@@ -104,8 +104,13 @@ step), is in `007/docs/loop-canvas.md`. Two hooks make that real, and **neither
 exists yet** — both are Floor-1 work, not current behaviour:
 
 - **007 side — `sandbox_policy` on `GateStep`.** A per-step policy path so the
-  gate runner knows to wrap the step. Forward-compatible with the current
-  manifest parser (unknown fields are tolerated), but **not yet added**.
+  gate runner knows to wrap the step. **Not yet added.** The manifest parser
+  tolerates unknown fields, but this is a **security control**, so it must
+  **fail closed** when it lands: a manifest `schema` bump (or explicit presence
+  check) so an older `o7` that can't enforce a `sandbox_policy` **refuses the
+  step** rather than silently running it bare under `bypassPermissions`. Relying
+  on unknown-field tolerance here would fail *open*. See
+  `007/docs/loop-canvas.md`.
 - **sandboy side — `--report <json>`.** A flag emitting enforcement status /
   exit code / duration, the machine-readable evidence the Observability field
   asks for. **Not implemented today:** `parse_args` (`src/main.rs`) accepts only