Stop stripping and normalizing generic Kimaki agent guidance after 0.13 fallback

[chubes4]

Problem

wp-coding-agents currently compensates for Kimaki generic --agent <current_agent> guidance in two places:

• dm-context-filter.ts strips generic agent override examples from the Kimaki system prompt.
• datamachine-kimaki rewrites any kimaki send --agent <anything> to --agent build.

Kimaki 0.13 now falls back to build when the requested agent does not exist, so this compensation may no longer be necessary and may hide useful generic Kimaki behavior.

Scope

• Verify Kimaki 0.13 fallback behavior in source and, where practical, with a focused runtime or unit test.
• Remove stripAgentOverrideInlines() and related --agent leak triggers if fallback makes them unnecessary.
• Remove or simplify datamachine-kimaki agent normalization if native fallback is enough.
• Keep any shim behavior still needed for Data Machine CLI-channel compatibility, but do not rewrite --agent just to compensate for old Kimaki behavior.
• Update docs/tests/snapshots.

Acceptance criteria

• Generic Kimaki agent guidance is no longer stripped solely because of nonexistent-agent concerns.
• datamachine-kimaki no longer rewrites --agent unless a separate current requirement is proven.
• Tests cover the new expected behavior, including Kimaki 0.13 fallback or an equivalent fixture.
• Documentation explains that Kimaki handles nonexistent agent fallback to build.

AI assistance

This issue was identified during an OpenCode GPT-5.5 review of wp-coding-agents against Kimaki 0.13.0, with follow-up clarification from Chris that Kimaki now falls back to build for nonexistent agents.