Refocus ctx-docs on architecture checkpoints

[Garabed96]

Summary

• Replace the Linear-first ctx-docs workflow with a single architecture checkpoint documentation mode
• Add evidence-gathering, boundary-language, verification, and Mermaid diagram guidance
• Update the README skill summary for ctx-docs

Verification

• git diff --cached --check
• basic structural Python check for both ctx-docs skill files

Notes

• Left unrelated local modification in plugins/ctx-codex/scripts/test-scripts.sh unstaged and uncommitted