[codex] focus public package for Codex OSS

[ryu-kraken]

Summary

This PR narrows the public package to the reusable Codex + Obsidian mechanism and keeps private vault contents out of scope.

Changes

• Refreshes README around the public Codex + Obsidian surface.
• Updates AGENTS.md for the public repo workflow.
• Removes CLAUDE.md and GEMINI.md from the public pitch.
• Adds a privacy boundary document.
• Adds a Codex for OSS application draft.
• Tightens .gitignore around private vault folders and generated local artifacts.

Validation

• Checked the branch diff is limited to the intended 7 public-facing files.
• Confirmed the branch was pushed to the ryu-kraken fork.