## Why
Session isolation relied on Guardian source attribution. An explicit policy lets callers control inherited capabilities independently of how a session is attributed.
## What changed
- Add `SessionIsolation` with default `Inherit` and opt-in `Isolated` modes, captured at session startup through `ExtensionDataInit`.
- Use the policy to control inherited instructions, extensions, execution rules, and MCP resources. Isolated sessions retain managed execution rules and omit executor-discovered MCP servers.
- Explicitly isolate Guardian reviewers while preserving source-based fallback for older callers and saved reviewers.
## Testing
Extend delegate tests to cover isolation independently of attribution, update execution-policy coverage to supply the explicit policy, and assert that managed reviewers do not inherit the parent's configured MCP tools.
GitOrigin-RevId: f0ab42fa2f0237860ec661e72693ab191216afbf
