fix(marketplace): use relative sources to avoid SSH-only clones

Both plugin entries in .claude-plugin/marketplace.json used absolute
GitHub sources (github / git-subdir), which trigger a second git clone
when a user runs `/plugin install`. That clone defaults to the SSH
transport and fails on machines without SSH keys or without
github.com in known_hosts:

    git@github.com: Permission denied (publickey).
    Host key verification failed.

The failure is common in corporate environments (HTTPS-only proxies)
and GitHub Codespaces / devcontainers (no SSH host keys preloaded),
and Claude Code surfaces it silently — the skill shows a red circle
with no actionable error. The marketplace itself clones fine because
Claude Code's marketplace loader falls back to HTTPS, but the plugin
install path does not.

Since both plugins live inside this same repo that hosts
marketplace.json, switch to relative sources so Claude Code reuses the
already-cloned marketplace checkout and never performs a second clone:

- `nx` → `"source": "./"` (plugin manifest at .claude-plugin/plugin.json)
- `polygraph` → `"source": "./generated/polygraph"` (manifest at
  generated/polygraph/.claude-plugin/plugin.json)

This matches the pattern already used by .cursor-plugin/marketplace.json
in this repo (`"source": "."`) and by vercel-labs/agent-browser, both of
which install cleanly in HTTPS-only and devcontainer environments.

Verified locally with:

    claude plugin validate .claude-plugin/marketplace.json  # ✔ passed
    claude plugin marketplace add <local-clone>
    claude plugin install nx@nx-claude-plugins              # ✔ installed
    claude plugin install polygraph@nx-claude-plugins       # ✔ installed

After install, all 7 nx skills (nx-workspace, nx-generate, nx-run-tasks,
nx-plugins, nx-import, monitor-ci, link-workspace-packages) and the
polygraph subagents/skills load as expected.

Fixes #106

Author: drewbitt

.claude-plugin/marketplace.json

@@ -11,19 +11,12 @@
     {
       "name": "nx",
       "description": "Claude Code plugin for Nx - helps with generators, documentation, and CI monitoring with self-healing integration",
-      "source": {
-        "source": "github",
-        "repo": "nrwl/nx-ai-agents-config"
-      }
+      "source": "./"
     },
     {
       "name": "polygraph",
       "description": "Coordinate changes across multiple repositories using Polygraph - delegate work to child agents, monitor CI across repos, and manage multi-repo sessions",
-      "source": {
-        "source": "git-subdir",
-        "url": "nrwl/nx-ai-agents-config",
-        "path": "generated/polygraph"
-      }
+      "source": "./generated/polygraph"
     }
   ]
 }