t_p6_api: build plugin scripting system

- Add PluginBase (plugin_base.gd) — abstract base class with 12 virtual hooks
- Add PluginManifest (plugin_manifest.gd) — custom Resource for plugin metadata
- Add PluginManager (plugin_manager.gd) — autoload singleton: directory scan,
  hook dispatch, lifecycle (load/unload/reload/rescan), cvar integration
- Add hello_world example plugin with all hooks demonstrating usage
- Register PluginManager autoload in project.godot
- Extend rcon_command_handler.gd with 'plugin' subcommand (list/load/unload/
  reload/info/rescan) delegating to PluginManager

server/scripts/plugin_api/plugin_manifest.gd

@@ -0,0 +1,80 @@
+## Plugin Manifest — Custom Resource
+##
+## Each plugin directory must contain a plugin.tres file of this type.
+## The PluginManager scans for these files when discovering plugins.
+##
+## Example plugin.tres:
+##   [gd_resource type="Resource" format=3]
+##   [ext_resource type="Script" id="1" path="res://server/scripts/plugin_api/plugin_manifest.gd"]
+##   [resource]
+##   script = ExtResource("1")
+##   name = "Hello World"
+##   version = "1.0.0"
+##   author = "Tactical Shooter Team"
+##   script_path = "hello_world.gd"
+##   enabled = true
+
+extends Resource
+class_name PluginManifest
+
+# ---------------------------------------------------------------------------
+# Exported fields
+# ---------------------------------------------------------------------------
+
+## Human-readable plugin name (e.g. "Hello World").
+## This is used as the canonical identifier for load/unload/info commands.
+@export var name: String = &""
+
+## Plugin version string (e.g. "1.0.0").
+## Follows semver convention for compatibility checking.
+@export var version: String = &"1.0.0"
+
+## Author name, handle, or team name.
+@export var author: String = &""
+
+## Path to the main GDScript file, relative to the plugin directory
+## (e.g. "hello_world.gd" for a script in the same directory).
+@export var script_path: String = &""
+
+## Whether the plugin is enabled on initial load.
+## Set to false to keep the plugin discovered but not auto-loaded.
+@export var enabled: bool = true
+
+## Minimum required PluginManager API version (semver string).
+## Leave empty if no minimum is required.
+@export var min_api_version: String = &""
+
+## Maximum allowed PluginManager API version (semver string).
+## Leave empty if any version is accepted.
+@export var max_api_version: String = &""
+
+## Arbitrary key-value metadata (e.g. description, homepage, dependencies).
+## Useful for plugin browsers and info commands.
+@export var metadata: Dictionary = {}
+
+# ---------------------------------------------------------------------------
+# Validation helpers
+# ---------------------------------------------------------------------------
+
+## Returns true if the manifest fields are minimally valid and usable.
+##
+## A valid manifest must have:
+##   - A non-empty name
+##   - A non-empty script_path referencing the plugin's main script
+func is_valid() -> bool:
+	return not name.is_empty() and not script_path.is_empty()
+
+## Returns true if this plugin is compatible with the given API version.
+## Uses simple string comparison — extend with proper semver logic as needed.
+func is_api_compatible(api_version: String) -> bool:
+	if not min_api_version.is_empty() and api_version < min_api_version:
+		return false
+	if not max_api_version.is_empty() and api_version > max_api_version:
+		return false
+	return true
+
+## Returns a human-readable summary of the manifest.
+func to_summary() -> String:
+	return "%s v%s by %s (script: %s, enabled: %s)" % [
+		name, version, author, script_path, str(enabled)
+	]