Require explicit memory modes for prompt injection

[chubes4]

Summary

• Treat memory files without explicit modes as registered/manageable but not prompt-injected.
• Keep prompt injection explicit by defaulting omitted modes to retrieval_policy: never and filtering non-eager files from get_for_mode().
• Add explicit MODE_ALL opt-ins for Data Machine core memory files that should remain injected, plus regression coverage for external runtime files.

Why

AGENTS.md and similar convention files exist for external runtimes with filesystem/WP-CLI access, not for AI prompt memory. Previously, omitted modes defaulted to all, so external-only files were injected into pipeline AI requests as noise.

Testing

• php -l inc/Engine/AI/MemoryFileRegistry.php
• php -l inc/bootstrap.php
• php -l tests/Unit/AI/Memory/MemoryPolicyResolverTest.php
• homeboy test data-machine currently fails one unrelated existing test: DataMachine\\Tests\\Unit\\Abilities\\ImageGenerationPromptRefinementTest::test_refine_prompt_includes_post_context_when_provided expects Article context: but receives an empty string. The memory policy tests passed in the full run.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Root-cause tracing, implementation, and test updates; Chris reviewed the architecture and directed the explicit opt-in behavior.