fix: use explicit git-subdir source for Cowork marketplace-sync compatibility

[isabelle-cedar]

Summary

• Cowork's marketplace "Add from a repository" flow fails with "Marketplace sync failed. Check the repository URL and try again." — reproduced live. Claude Code works fine with the same repo.
• Root cause: our marketplace.json used the relative-path shorthand ("source": "./cedar-plugin"), which only resolves when the client does a full local git clone of the whole repo (what Claude Code's CLI does). Cowork's server-side remoteMarketplaceOps sync validates each plugin's source independently via an unauthenticated server-side fetch instead of a full clone, and can't resolve a bare relative path — it reports a generic sync failure. This matches the server-side log dump quoted in anthropics/claude-code#61271 (closed as duplicate of the still-open #28125): "Repository not found on github.com. Check the URL and make sure the repository is public."
• The official docs' own troubleshooting section documents this exact failure class ("Plugins with relative paths fail in URL-based marketplaces") and its fix: use an explicit source object instead of a relative path.

Fix

Changed the cedar plugin's source from the relative-path shorthand to an explicit git-subdir source (required instead of github, since github has no path field and our plugin lives in the cedar-plugin/ subdirectory of this repo, not at the root):

-      "source": "./cedar-plugin",
+      "source": {
+        "source": "git-subdir",
+        "url": "https://github.com/CedarCopilot/cedar-plugin.git",
+        "path": "cedar-plugin"
+      },

Test plan

• claude plugin validate . passes (pre-existing description warning only, unrelated)
• JSON validity + CI's dev-artifact guard unaffected (only checks .mcp.json)
• Re-test in Cowork: /plugin marketplace remove the old entry, re-add via CedarCopilot/cedar-plugin, confirm sync succeeds
• Re-verify Claude Code install still works unchanged (git-subdir is also a fully-supported source type there)

🤖 Generated with Claude Code