Fix effective prompt contract drift

[chubes4]

Summary

Repair the managed Kimaki effective-prompt contract after the installed Kimaki prompt changed from the generic prompt to the compact managed prompt.

What changed

• Replace only positively identified Kimaki prompt content instead of globally stripping generic section headings from composed system/message blocks.
• Preserve unrelated composed guidance, including the generic WordPress read-only source contract.
• Make the plugin and post-upgrade patch emit the same literal $HOME/.kimaki/kimaki.log managed prompt text.
• Normalize message-transform snapshot endings so system and message transforms compare the same effective text.
• Refresh reviewed snapshots to the current 726-character managed prompt and add executable regression coverage for the patched prompt.

How to test

• node tests/effective-prompt/run.mjs
• ./tests/kimaki-system-message-patch.sh
• ./tests/kimaki-agent-fallback.sh
• ./tests/kimaki-managed-plugin-rig.sh

All targeted checks passed. All 13 GitHub Actions checks passed.

Evidence

• Fixes Repair Kimaki effective-prompt transform drift #296
• Homeboy cook: agent-task-bfaff85f-ecbb-4603-ac48-cc13efc2248d
• Adopted gate/finalization attempt: agent-task-bfaff85f-ecbb-4603-ac48-cc13efc2248d-attempt-2-8f78f6f7
• Verified base: c00bbe84cfa0d68181ad638128c8b0aabb00fda6
• Candidate: ee2a45fc41af8270f490714a79df0a54b1e2ea49

AI assistance

• Tool: Homeboy with OpenCode
• Model: openai/gpt-5.6-terra
• Used for: Diagnosing the transform drift, implementing the filter and prompt-patch changes, updating high-signal regression coverage and reviewed snapshots, and preparing the pull request. Chris reviewed the diff and reran the targeted tests before finalization.