Promote snippets and prompts

[huangruiteng]

Summary

• Promote Notes/snippets/ into top-level snippets/ and add a top-level prompts/ library beside Notes/.
• Migrate reusable prompt assets out of the old snippet bucket, remove the generic Notes/snippets/prompt.py, and update CS-Notes workflow references.
• Add recent agent/product/process notes plus Codex builder Xiaohongshu publishing assets.

Commit Grouping

1. chore: promote snippets and prompts - top-level directory migration, path updates, prompt classification, and one private internal doc URL redaction from a moved snippet.
2. notes: add agent product and process insights - Codex product work notes, CUA single-task vs long-running value delivery, and waterfall development notes.
3. writing: add Codex builder post assets - post draft, page spec, and generated image assets.

Validation

• git diff --check HEAD~3..HEAD
• python3 snippets/check-snippet-links.py
• python3 -m py_compile snippets/check-snippet-links.py snippets/markdown_toc.py snippets/codex_todo_triage.py snippets/todo-prompt.py snippets/todo_utils.py snippets/material-candidates-healthcheck.py snippets/note-section-healthcheck.py
• python3 snippets/markdown_toc.py Notes/AI-Agent-Product\&PE.md
• python3 -m json.tool 创作/Codex负责人builder观点-小逸排版/spec.json
• Staged sensitive scan for private doc URLs / token-shaped values; redacted the one internal Lark doc URL found in a moved snippet.

Excluded Local/Private Artifacts

• Ignored .DS_Store and sample .env files under moved directories were not staged.
• No .local/ files were staged.

Residual Risk

• Some legacy launchd helper templates still include the user-local CS-Notes path as their default install target. They predate this PR and contain no credentials; a future cleanup can parameterize them if desired.