Rename repo `akd-plugins-claude` to `akd-plugins` and update README

Summary

Rename this repository from akd-plugins-claude to akd-plugins and update the README to match.

Motivation (WHY)

Drop the -claude suffix so the name is simpler and not tied to a single surface as AKD plugins expand.

Description (WHAT)

• Rename the repository akd-plugins-claude → akd-plugins.
• Update README references to the new name — including the /plugin marketplace add NASA-IMPACT/akd-plugins-claude command.
• Update any other in-repo references (configs, docs, paths, badges).

Acceptance Criteria (DONE =)

• Repo is named akd-plugins.
• README reflects the new name everywhere — no stale akd-plugins-claude references remain.
• Plugin marketplace install instructions work against the new name.

Note: GitHub auto-redirects the old repo URL after a rename, but hardcoded references (e.g. the marketplace add command) should still be updated.

Planning / parent: NASA-IMPACT/akd-core#488 (sub-issue of NASA-IMPACT/akd-core#482, under EPIC NASA-IMPACT/akd-core#480).

Rename the repository from akd-plugins-claude to akd-plugins, then search the repository for the old name, starting with README references and the plugin marketplace add command. Update the README and any other in-repo references, including configs, docs, paths, and badges; done means no stale references remain and the install instructions use the new name.