docs: add shared coding-agent guidance

[binaryaaron]

What changed and why

Add canonical coding-agent instructions, a thin Claude adapter and shared/private file guidance. Extend existing exclusion examples to agent files and worktrees.

Stack position: 12/12. Base: codex/usersim-draft-pr11-mise. Merge in stack order.

How it was verified

Documentation checks and existing agent/exclusion regressions cover the retained incremental guidance. Final product bytes match the independently reviewed simplification source.

The latest cleanup used focused checks and independent source review. Earlier full matrix/native/container results retain their original revision; those broad runs and all twelve Arc reviews were not repeated after the cleanup. GitHub CI results are pending.

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