feat(tui): full keyboard support for config TUI and chat slash completion

Add Header/Footer with visible key hints, ctrl+right/ctrl+left tab navigation,
ctrl+s save bindings for General and plugin config tabs, e/d bindings for
plugin enable/disable in the Plugins tab. Extract shared _do_save() and
_toggle_plugin() helpers so button and key paths share one code path.

Add WordCompleter to the chat REPL so Tab completes slash commands.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pyra/chat/session.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 import litellm
 from prompt_toolkit import PromptSession
+from prompt_toolkit.completion import WordCompleter
 from prompt_toolkit.history import FileHistory
 
 from pyra.chat.history import ConversationHistory
@@ -159,12 +160,15 @@ def start_chat() -> None:
     ))
 
     history = ConversationHistory(cfg, registry)
-    session: PromptSession = PromptSession(
-        history=FileHistory(str(_HISTORY_FILE)),
-        multiline=False,
-    )
 
     plugin_slash = registry.get_slash_commands()
+    all_commands = list(_STATIC_COMMANDS) + list(plugin_slash)
+    session: PromptSession = PromptSession(
+        history=FileHistory(str(_HISTORY_FILE)),
+        completer=WordCompleter(all_commands, sentence=True),
+        complete_while_typing=False,
+        multiline=False,
+    )
 
     provider = get_provider(cfg.ai.provider_id)
     render_system(