[docs] Add security boundaries reference in SECURITY.md (#17848)

## Summary
1. Add a Security Boundaries section to `SECURITY.md`.
2. Point readers to the Codex Agent approvals and security documentation
for sandboxing, approvals, and network controls.

## Validation
1. Reviewed the `SECURITY.md` diff in a clean worktree.
2. No tests run. Docs only change.

codex-rs/core/BUILD.bazel

@@ -1,4 +1,4 @@
-load("//:defs.bzl", "codex_rust_crate")
+load("//:defs.bzl", "codex_rust_crate", "workspace_root_test")
 
 filegroup(
     name = "model_availability_nux_fixtures",
@@ -31,6 +31,15 @@ codex_rust_crate(
         "//codex-rs/apply-patch:apply_patch_tool_instructions.md",
         "templates/realtime/backend_prompt.md",
     ],
+    integration_test_flaky_by_stem = {
+        "all": [
+            "suite::abort_tasks::interrupt_tool_records_history_entries",
+            "suite::apply_patch_cli::apply_patch_cli_rejects_invalid_hunk_header::applypatchmodeloutput_function_expects",
+        ],
+        "responses_headers": [
+            "responses_stream_includes_turn_metadata_header_for_git_workspace_e2e",
+        ],
+    },
     integration_test_timeout = "long",
     test_data_extra = [
         "config.schema.json",
@@ -48,6 +57,12 @@ codex_rust_crate(
         "//:AGENTS.md",
     ],
     test_tags = ["no-sandbox"],
+    unit_test_args = [
+        "--skip",
+        "guardian::tests::guardian_parallel_reviews_fork_from_last_committed_trunk_history",
+        "--skip",
+        "agent::control::tests::completion_watcher_notifies_parent_when_child_is_missing",
+    ],
     unit_test_timeout = "long",
     extra_binaries = [
         "//codex-rs/linux-sandbox:codex-linux-sandbox",
@@ -57,3 +72,41 @@ codex_rust_crate(
         "//codex-rs/cli:codex",
     ],
 )
+
+workspace_root_test(
+    name = "core-guardian-parallel-reviews-flaky-test",
+    env = {
+        "INSTA_WORKSPACE_ROOT": ".",
+        "INSTA_SNAPSHOT_PATH": "src",
+    },
+    test_bin = ":core-unit-tests-bin",
+    workspace_root_marker = "//codex-rs/utils/cargo-bin:repo_root.marker",
+    args = [
+        "--exact",
+        "guardian::tests::guardian_parallel_reviews_fork_from_last_committed_trunk_history",
+    ],
+    tags = [
+        "flaky",
+        "no-sandbox",
+    ],
+    timeout = "long",
+)
+
+workspace_root_test(
+    name = "core-completion-watcher-missing-child-flaky-test",
+    env = {
+        "INSTA_WORKSPACE_ROOT": ".",
+        "INSTA_SNAPSHOT_PATH": "src",
+    },
+    test_bin = ":core-unit-tests-bin",
+    workspace_root_marker = "//codex-rs/utils/cargo-bin:repo_root.marker",
+    args = [
+        "--exact",
+        "agent::control::tests::completion_watcher_notifies_parent_when_child_is_missing",
+    ],
+    tags = [
+        "flaky",
+        "no-sandbox",
+    ],
+    timeout = "long",
+)