feat(agent): add hardened MCP toolchain shims with config isolation

Add hermit-managed shim scripts for uv/uvx/npx/node that provide
config-isolated toolchain execution for MCP servers, preventing
ambient system toolchains from leaking user config into agent
processes.

Security hardening:
- Private mktemp temp dir with cleanup trap (no predictable paths)
- Lock mutual exclusion via kill-0 + /proc start-time verification
- Atomic bootstrap recovery (staged download + validated mv)
- Desktop fail-loud validation of shim resources before env export

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

scripts/run-tests.sh

@@ -100,6 +100,12 @@ run_unit_tests() {
 
   run_test_step "buzz-push-gateway tests" \
     cargo test -p buzz-push-gateway -- --nocapture
+
+  # MCP shim hostile-config suite: infra-free (real wrapper scripts +
+  # stubbed curl/openssl). Contains the Linux-only hostile-path regression
+  # for the CRITICAL temp-execution fix — this is its only CI execution path.
+  run_test_step "buzz-agent shim hostile-config tests" \
+    cargo test -p buzz-agent --test shim_hostile_config -- --nocapture
 }
 
 # ---- DB / integration tests (infra required) --------------------------------