feat: integrate AirLLM as optional high-performance backend

Adds the `bigbrain` optional dependency group (airllm>=2.9.0) and a
complete second inference path that runs 8B / 70B / 405B Llama models
locally via layer-by-layer loading — no GPU required, no cloud, fully
sovereign.

Key changes:
- src/timmy/backends.py   — TimmyAirLLMAgent (same print_response interface
                            as Agno Agent); auto-selects AirLLMMLX on Apple
                            Silicon, AutoModel (PyTorch) everywhere else
- src/timmy/agent.py      — _resolve_backend() routing with explicit override,
                            env-config, and 'auto' Apple-Silicon detection
- src/timmy/cli.py        — --backend / --model-size flags on all commands
- src/config.py           — timmy_model_backend + airllm_model_size settings
- src/timmy/prompts.py    — mentions AirLLM "even bigger brains, still fully
                            sovereign"
- pyproject.toml          — bigbrain optional dep; wheel includes updated
- .env.example            — TIMMY_MODEL_BACKEND + AIRLLM_MODEL_SIZE docs
- tests/conftest.py       — stubs 'airllm' module so tests run without GPU
- tests/test_backends.py  — 13 new tests covering helpers + TimmyAirLLMAgent
- tests/test_agent.py     — 7 new tests for backend routing
- README.md               — Big Brain section with one-line install
- activate_self_tdd.sh    — bootstrap script (venv + install + tests +
                            watchdog + dashboard); --big-brain flag

All 61 tests pass. Self-TDD watchdog unaffected.

https://claude.ai/code/session_01DMjQ5qMZ8iHeyix1j3GS7c

src/timmy/cli.py

@@ -1,3 +1,5 @@
+from typing import Optional
+
 import typer
 
 from timmy.agent import create_timmy
@@ -5,25 +7,50 @@ from timmy.prompts import TIMMY_STATUS_PROMPT
 
 app = typer.Typer(help="Timmy — sovereign AI agent")
 
+# Shared option definitions (reused across commands for consistency).
+_BACKEND_OPTION = typer.Option(
+    None,
+    "--backend",
+    "-b",
+    help="Inference backend: 'ollama' (default) | 'airllm' | 'auto'",
+)
+_MODEL_SIZE_OPTION = typer.Option(
+    None,
+    "--model-size",
+    "-s",
+    help="AirLLM model size when --backend airllm: '8b' | '70b' | '405b'",
+)
+
 
 @app.command()
-def think(topic: str = typer.Argument(..., help="Topic to reason about")):
+def think(
+    topic: str = typer.Argument(..., help="Topic to reason about"),
+    backend: Optional[str] = _BACKEND_OPTION,
+    model_size: Optional[str] = _MODEL_SIZE_OPTION,
+):
     """Ask Timmy to think carefully about a topic."""
-    timmy = create_timmy()
+    timmy = create_timmy(backend=backend, model_size=model_size)
     timmy.print_response(f"Think carefully about: {topic}", stream=True)
 
 
 @app.command()
-def chat(message: str = typer.Argument(..., help="Message to send")):
+def chat(
+    message: str = typer.Argument(..., help="Message to send"),
+    backend: Optional[str] = _BACKEND_OPTION,
+    model_size: Optional[str] = _MODEL_SIZE_OPTION,
+):
     """Send a message to Timmy."""
-    timmy = create_timmy()
+    timmy = create_timmy(backend=backend, model_size=model_size)
     timmy.print_response(message, stream=True)
 
 
 @app.command()
-def status():
+def status(
+    backend: Optional[str] = _BACKEND_OPTION,
+    model_size: Optional[str] = _MODEL_SIZE_OPTION,
+):
     """Print Timmy's operational status."""
-    timmy = create_timmy()
+    timmy = create_timmy(backend=backend, model_size=model_size)
     timmy.print_response(TIMMY_STATUS_PROMPT, stream=False)