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_base.gd

@@ -0,0 +1,158 @@
+## Plugin Base — Abstract plugin interface
+##
+## All server plugins must extend this class and may override any
+## of the virtual hook methods.  Hooks that return a bool act as
+## filters/callbacks — return true to allow/accept the event,
+## false to block/reject it (where the hook definition supports it).
+##
+## The base class provides a `plugin_name` property that is
+## automatically populated at load time from the manifest, and a
+## `log(msg)` convenience method for structured logging.
+##
+## Usage:
+##   extends PluginBase
+##   func on_server_start():
+##       log_info("My plugin started!")
+##
+##   func on_player_say(id, msg) -> bool:
+##       if "badword" in msg:
+##           return false  # block the message
+##       return true
+
+extends Node
+class_name PluginBase
+
+# ---------------------------------------------------------------------------
+# Public properties — populated by PluginManager at load time
+# ---------------------------------------------------------------------------
+
+## The canonical name of this plugin, set from the manifest at load time.
+var plugin_name: String = &""
+
+## The version string from the manifest (e.g. "1.0.0").
+var plugin_version: String = &""
+
+## The author string from the manifest.
+var plugin_author: String = &""
+
+# ---------------------------------------------------------------------------
+# Logging helpers
+# ---------------------------------------------------------------------------
+
+## Log an informational message prefixed with the plugin name.
+func log_info(msg: String) -> void:
+	print("[Plugin:%s] %s" % [plugin_name, msg])
+
+## Log a warning message prefixed with the plugin name.
+func log_warn(msg: String) -> void:
+	push_warning("[Plugin:%s] %s" % [plugin_name, msg])
+
+## Log an error message prefixed with the plugin name.
+func log_err(msg: String) -> void:
+	push_error("[Plugin:%s] %s" % [plugin_name, msg])
+
+## Set metadata on this plugin from the manifest.
+## Called automatically by PluginManager during load.
+func set_plugin_meta(name: String, version: String, author: String) -> void:
+	plugin_name = name
+	plugin_version = version
+	plugin_author = author
+
+# ---------------------------------------------------------------------------
+# Server lifecycle hooks
+# ---------------------------------------------------------------------------
+
+## Called when the server starts (after all autoloads are ready).
+## Use this to initialise timers, connect signals, or register cvars.
+func on_server_start() -> void:
+	pass
+
+## Called when the server is shutting down.
+## Use this to flush state, save data, or disconnect from external services.
+func on_server_stop() -> void:
+	pass
+
+# ---------------------------------------------------------------------------
+# Map lifecycle hooks
+# ---------------------------------------------------------------------------
+
+## Called when a map starts loading.
+## map_name: the resource path or identifier of the map.
+func on_map_start(map_name: String) -> void:
+	pass
+
+## Called when a map ends (all players left, next map loading).
+## map_name: the resource path or identifier of the map that ended.
+func on_map_end(map_name: String) -> void:
+	pass
+
+# ---------------------------------------------------------------------------
+# Player hooks
+# ---------------------------------------------------------------------------
+
+## Called when a player attempts to connect to the server.
+## id: the peer/connection id assigned by the network layer.
+## name: the player's display name.
+## steam_id: the player's unique platform identifier.
+##
+## Return true to allow the connection, false to reject it.
+func on_player_connect(id: int, name: String, steam_id: String) -> bool:
+	return true
+
+## Called when a player disconnects from the server.
+## id: the peer/connection id of the player.
+## reason: a string describing why the player left (optional).
+func on_player_disconnect(id: int, reason: String) -> void:
+	pass
+
+## Called when a player sends a chat message.
+## id: the peer/connection id of the sender.
+## msg: the plain-text chat message.
+##
+## Return true to allow the message through, false to suppress it.
+func on_player_say(id: int, msg: String) -> bool:
+	return true
+
+## Called when a player spawns into the game world.
+## id: the peer/connection id of the player.
+func on_player_spawn(id: int) -> void:
+	pass
+
+## Called when a player is killed.
+## victim: peer id of the killed player.
+## killer: peer id of the killer (may equal victim for suicide).
+## weapon: weapon name/identifier string (e.g. "rifle", "pistol").
+func on_player_killed(victim: int, killer: int, weapon: String) -> void:
+	pass
+
+# ---------------------------------------------------------------------------
+# Round hooks
+# ---------------------------------------------------------------------------
+
+## Called when a new round begins.
+## number: the round number (1-based, increments each round).
+func on_round_start(number: int) -> void:
+	pass
+
+## Called when a round ends.
+## winner_team: 0 = draw/stalemate, 1 = team A, 2 = team B.
+## reason: a string describing the end condition ("time_limit", "all_dead", etc.).
+func on_round_end(winner_team: int, reason: String) -> void:
+	pass
+
+## Called when the team scores change.
+## team_a_score and team_b_score reflect the current match state.
+func on_score_changed(team_a_score: int, team_b_score: int) -> void:
+	pass
+
+# ---------------------------------------------------------------------------
+# Per-tick hook
+# ---------------------------------------------------------------------------
+
+## Called every physics tick (at the configured tick rate, typically 128 Hz).
+## delta: time elapsed since last tick in seconds (approximately 1/tickrate).
+##
+## WARNING: This is called at high frequency — keep implementations cheap
+## to avoid impacting server performance.
+func on_tick(delta: float) -> void:
+	pass