fix(agent-guard): unify agent_guard import style in test_opencode.py

[HeaTTap]

Fixes #955

Unified the import statement for agent_guard in tools/agent-guard/tests/test_opencode.py to use from agent_guard import ... consistent with test_kiro.py.

[justinmclean]

The change is sound. After it, agent_guard has no remaining attribute references in the file, so dropping import agent_guard leaves nothing dangling. All five names on the from-import are used: ALLOW_EXIT (6), DENY_EXIT (4), cli (3), opencode_main (8), dispatch (3). Nothing in the file monkeypatches the module object, so the module binding was genuinely only there for the one agent_guard.dispatch(...) call.