docs: move sandbox threat model to docs/security.md, add code reference

CLAUDE.md

@@ -10,6 +10,8 @@ when you need depth on a subsystem. This file is the index.
 - Operator/agent trust-boundary design:
   **[docs/boundary.md](docs/boundary.md)** (`area:ops` issues
   for the deployment/gateway/privsep work).
+- Credential isolation + sandbox threat model:
+  **[docs/security.md](docs/security.md)**.
 
 ## File map
 

docs/security.md

@@ -0,0 +1,33 @@
+# Security model
+
+## Nix builds and credential isolation (issue #240)
+
+### Background
+
+Agent containers bind-mount the host's `nix-daemon` socket. The host daemon may
+have `sandbox-fallback = false` (strict NixOS defaults), which causes `nix build`
+inside nspawn containers to fail — containers lack kernel user namespaces, so nix
+cannot set up its build sandbox. `harness-base.nix` sets `sandbox-fallback = true`
+so that builds fall back to unsandboxed execution rather than failing outright.
+
+### Threat model
+
+Unsandboxed nix builds run as `nixbld` users (non-root, typically UIDs 30001-30010).
+Without sandbox isolation, a build derivation's builder script has read access to
+any file in the container that the nixbld user can read.
+
+**What is NOT exposed**:
+
+- `/root/.claude/` — mode `0700`, owned by root. nixbld users cannot read it.
+- `/state/forge-token` — written at mode `0600` by `hive-c0re/src/forge.rs`.
+  nixbld users cannot read it.
+
+**Policy**: all credential files written to agent state directories MUST be mode
+`0600` or stricter. Do not create world-readable secret files in agent state dirs.
+
+### Long-term fix
+
+The proper fix is to enable user namespaces inside nspawn containers
+(`--private-users=inherit` in `EXTRA_NSPAWN_FLAGS`) so nix can set up its real
+sandbox and `sandbox-fallback` becomes a true last resort. This requires verifying
+bind-mount compatibility with user namespace UID mapping and is tracked as a TODO.

nix/templates/harness-base.nix

@@ -353,19 +353,7 @@
     # fail inside the container. Enable sandbox-fallback so builds that
     # can't set up the sandbox (no user-namespaces in nspawn) fall back
     # to unsandboxed local builds rather than failing outright.
-    #
-    # Security note (issue #240): unsandboxed builds run as `nixbld` users
-    # (non-root) and can read any world-readable file in the container.
-    # Mitigations in place:
-    #   - /root/.claude (claude session) is mode 700, root-owned - nixbld
-    #     users cannot access it.
-    #   - /state/forge-token (Forgejo API token) is written at mode 0600 by
-    #     hive-c0re/src/forge.rs - nixbld users cannot access it.
-    # All future credential files written to agent state dirs MUST be mode
-    # 0600 or stricter. Do NOT create world-readable secret files.
-    # Long-term fix: enable user namespaces in nspawn containers
-    # (--private-users=inherit) so the nix sandbox actually works and
-    # sandbox-fallback becomes a true last resort.
+    # Security implications: see docs/security.md.
     nix.settings.sandbox-fallback = true;
 
     # `claude-code` is unfree. Each per-agent container's nixosConfiguration