[codex] package datamagic skill

[MisterBrookT]

Summary

• add docs CI and repo hygiene files
• move datamagic-video into skills/datamagic-video
• add Claude/Codex plugin metadata and one-line shell installer
• update README install links and commands

Notes

• experiments/ was left untracked and excluded from this PR
• branch contains earlier docs CI commits plus the skill packaging commit

Checks

• bash -n install.sh
• jq empty .claude-plugin/plugin.json .claude-plugin/marketplace.json .codex-plugin/plugin.json
• git diff --cached --check

[MisterBrookT]

Addressed Claude install docs.

• Replaced nonexistent /plugin marketplace add and /plugin install slash commands with Claude CLI commands:
  • claude plugin marketplace add HKUSTDial/DataMagic
  • claude plugin install datamagic-video@datamagic
• Kept Codex CLI commands unchanged.
• install.sh already used Claude CLI commands, so no script change needed.

Checks rerun:

• bash -n install.sh
• jq empty .claude-plugin/plugin.json .claude-plugin/marketplace.json .codex-plugin/plugin.json
• git diff --check