Remove automatic parameter namespacing from permission plugins

Simplifies the permission system by removing automatic parameter namespacing.
Plugins are now responsible for using unique parameter names. The recommended
convention is to prefix parameters with the plugin source name (e.g.,
:myplugin_user_id). System reserves :actor, :actor_id, :action, :filter_parent.

- Remove _namespace_params() function from datasette/utils/permissions.py
- Update build_rules_union() to use plugin params directly
- Document parameter naming convention in plugin_hooks.rst
- Update example plugins to use prefixed parameters
- Add test_multiple_plugins_with_own_parameters() to verify convention works

docs/plugin_hooks.rst

@@ -1461,8 +1461,12 @@ Return value
 
 Datasette's action-based permission resolver calls this hook to gather SQL rows describing which
 resources an actor may access (``allow = 1``) or should be denied (``allow = 0``) for a specific action.
-Each SQL snippet should return ``parent``, ``child``, ``allow`` and ``reason`` columns. Any bound parameters
-supplied via ``PluginSQL.params`` are automatically namespaced per plugin.
+Each SQL snippet should return ``parent``, ``child``, ``allow`` and ``reason`` columns.
+
+**Parameter naming convention:** Plugin parameters in ``PluginSQL.params`` should use unique names
+to avoid conflicts with other plugins. The recommended convention is to prefix parameters with your
+plugin's source name (e.g., ``myplugin_user_id``). The system reserves these parameter names:
+``:actor``, ``:actor_id``, ``:action``, and ``:filter_parent``.
 
 
 Permission plugin examples
@@ -1531,10 +1535,10 @@ will pass through to the SQL snippet.
                     1 AS allow,
                     'execute-sql allowed for analytics_*' AS reason
                 FROM catalog_databases
-                WHERE database_name LIKE :prefix
+                WHERE database_name LIKE :analytics_prefix
             """,
             params={
-                "prefix": "analytics_%",
+                "analytics_prefix": "analytics_%",
             },
         )
 
@@ -1564,12 +1568,12 @@ with columns ``(actor_id, action, parent, child, allow, reason)``.
                     allow,
                     COALESCE(reason, 'permission_grants table') AS reason
                 FROM permission_grants
-                WHERE actor_id = :actor_id
-                  AND action = :action
+                WHERE actor_id = :grants_actor_id
+                  AND action = :grants_action
             """,
             params={
-                "actor_id": actor.get("id"),
-                "action": action,
+                "grants_actor_id": actor.get("id"),
+                "grants_action": action,
             },
         )