feat: LLM hot-swap voice command + Ollama backend (148 → 151 commands)

Closes the last functional parity gap with rust. Python now has voice-driven
Ollama↔Groq switching, persistent across restarts.

llm_backend.py (new, ~130 lines)
  Singleton module:
    current_client()     → active OpenAI client (or None)
    current_backend()    → 'groq' | 'ollama' | 'none'
    current_model()      → active model name
    swap_to(name)        → hot-swap + persist to <here>/llm_backend.txt
    parse_backend(name)  → ru/en alias normaliser ('облако'→groq, 'локальный'→ollama)

  Backends:
    - Groq:   config.GROQ_TOKEN + GROQ_BASE_URL + GROQ_MODEL (defaults)
    - Ollama: OLLAMA_BASE_URL (default http://localhost:11434/v1) +
              OLLAMA_MODEL (default qwen2.5:3b). api_key='ollama' (placeholder,
              openai lib insists on a non-empty string; Ollama ignores it).

  Init precedence (idempotent _ensure_init):
    1. Persisted choice from llm_backend.txt
    2. JARVIS_LLM env var
    3. Auto-detect: Groq if token present, else Ollama

Voice commands (commands.yaml, +3 entries → 151)
  llm_switch_local  → ollama
  llm_switch_cloud  → groq
  llm_status        → speaks current backend

extensions.py
  + do_llm_switch / do_llm_status handlers
  + llm_backend import
  do_interesting_fact migrated off direct OpenAI(...) to use llm_backend.current_*

dev_handlers.py
  + llm_backend import
  do_codebase_ask + do_github_summarize_pr migrated to llm_backend.current_*

vision_handler.py
  Vision call documented as Groq-specific (Ollama doesn't expose vision via
  OpenAI-compat in our stack), kept direct config.GROQ_TOKEN reading.

Tests: ast.parse passes for all 5 modules. yaml.safe_load = 151 entries.

PYTHON PARITY VS RUST IS NOW FUNCTIONALLY COMPLETE.
Only remaining rust-only feature: the Tauri GUI (python is console-only).

main.py

@@ -1626,6 +1626,10 @@ def run_action(action):
         extensions.do_github_list_prs(action, _current_voice or '')
     elif t == 'github_summarize_pr':
         extensions.do_github_summarize_pr(action, _current_voice or '')
+    elif t == 'llm_switch':
+        extensions.do_llm_switch(action, _current_voice or '')
+    elif t == 'llm_status':
+        extensions.do_llm_status(action, _current_voice or '')
 
 
 _current_voice: str = ''