fix(agy): replace unreliable plugins.json discovery with a real installer

Testing against a live agy session confirmed the plugins.json "entries"
mechanism documented for agy does not reliably surface a plugins/<name>/
bundle as a plugin: hooks and MCP servers inside it are never loaded, and
only a stray skill occasionally surfaces via agy's generic skill-walk
("No plugins are currently loaded or discoverable in the active context").

Add scripts/install_agy.py, which instead pushes a generated dist/agy
into the locations confirmed to actually work: symlinks each skill into
~/.gemini/config/skills/<name>/ (or a project's .agents/skills/ with
--project) and merges each plugin's mcpServers/hooks into the shared
global mcp_config.json/hooks.json without touching unrelated entries.
Supports --uninstall to cleanly remove exactly what was added.
generate_plugins.py's --install-local now shells out to it for the agy
side instead of printing the broken plugins.json instructions.

Also fixes plugins/core-essentials/mcp.json, which had been hand-added
directly under the repo root (never discovered by the generator) with
invalid JSON (missing the outer object) and no env-var indirection for
its GitHub/Gitea tokens; moved under the plugin and wired up with
${VAR} placeholders instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014YVWWAnvR5TDQzmfY3RXG9

scripts/generate_plugins.py

@@ -413,8 +413,15 @@ def install_local(manifest: dict, out_dir: Path) -> None:
 
     agy_src = out_dir / "dist" / "agy"
     if agy_src.exists():
-        print("For agy, add this entry to ~/.gemini/config/plugins.json:")
-        print(json.dumps({"entries": [{"path": str(agy_src)}]}, indent=2))
+        install_agy_script = ROOT / "scripts" / "install_agy.py"
+        result = subprocess.run(
+            [sys.executable, str(install_agy_script), "--all", "--dist-dir", str(agy_src)],
+            capture_output=True,
+            text=True,
+        )
+        print(result.stdout, end="")
+        if result.returncode != 0:
+            print(result.stderr, end="", file=sys.stderr)
 
 
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