docs: update references for skills/plugins repo renames

[devantler]

What

The sibling repos devantler-tech/skills and devantler-tech/plugins were renamed to devantler-tech/agent-skills and devantler-tech/agent-plugins. This updates every stale reference to the old names across the docs and marketplace manifests.

Why

GitHub redirects renamed repos, but in-repo references should point at the canonical new names so links, install commands, and the upstream-skill provenance stay accurate and self-consistent.

Changes

• README.md — title, VS Code chat.plugins.marketplaces entry, Copilot CLI copilot plugin marketplace browse, Claude Code /plugin marketplace add, and the upstream skill-index link.
• AGENTS.md — title, upstream/sibling-repo links, the issue #7 link, and the repo-rename note (copilot-plugins → agent-plugins).
• .github/copilot-instructions.md — title and upstream-repo references.
• .github/plugin/marketplace.json + .claude-plugin/marketplace.json — metadata.description upstream reference. Both manifests edited together; they remain byte-for-byte in parity (jq -S diff is empty) and valid JSON.

Intentionally NOT changed

• Generic CLI commands (gh skill install, gh skill update --all, copilot plugin, /plugin install) — these are real tool commands, not repo references.
• The devantler-plugins marketplace name and the @devantler-plugins install suffix — these are the marketplace identifier, not the repo slug.
• The plugins/ and <plugin>/skills/ concept directories — structural names referenced by relative ./plugins/<name> paths, unrelated to the repo rename.

Validation

• grep over the whole repo confirms zero remaining devantler-tech/skills / devantler-tech/plugins references.
• Both manifests are valid JSON and remain in parity (CI's jq -S diff gate passes locally).

Generated with assistance from Claude Code.