docs(hooks): add immutable escrow hook risk warnings

[dev-jodee]

Summary

• add a dedicated hook warning section to README covering immutable escrow risk
• add warning language to Program Overview for SetHook and Hook extension behavior
• update security considerations to call out hook liveness dependency and HookRejected
• add UI warning copy to Set Hook and Set Immutable forms
• add an in-code comment documenting fail-closed hook invocation semantics

Test Plan

• pnpm install
• just generate-clients
• just build (needed in this environment to produce SBF artifacts)
• just fmt (fails on existing repository-wide ESLint baseline: 86 @typescript-eslint/no-unsafe-* errors across web/demo files unrelated to this PR)

Breaking Changes

• none