Version shared Claude and Codex configuration

[akan72]

Summary

• Manage portable Claude Code and Codex settings while preserving machine-local configuration.
• Add shared agent instructions and isolated synchronization tests.
• This PR is merged and now provides the Claude attribution and statusline behavior that supersedes Turn off Claude Code attribution (append-only) #24 and Preserve Claude statusline configuration #37.

Test Plan

• Ran tests/claude-sync-settings.sh, tests/codex-sync-config.sh, and tests/assimilate-path.sh locally and on the Mac mini.
• Ran merged main in a fresh synthetic home on the Mac mini and verified attribution, statusline, shared instructions, and preserved unmanaged settings.
• Ran bash -n assimilate.sh claude/sync-settings.sh codex/sync-config.sh tests/*.sh, jq -e . claude/settings.managed.json, and git diff --check.
• Screenshots: not applicable; validation was CLI-based.