[loop-cycle-40] fix: use get_system_prompt() in cloud backends (#135) (#138)

## What

Cloud backends (Grok, Claude, AirLLM) were importing SYSTEM_PROMPT directly, which is always SYSTEM_PROMPT_LITE and contains unformatted {model_name} and {session_id} placeholders.

## Changes

- backends.py: Replace `from timmy.prompts import SYSTEM_PROMPT` with `from timmy.prompts import get_system_prompt`
- AirLLM: uses `get_system_prompt(tools_enabled=False, session_id="airllm")` (LITE tier, correct)
- Grok: uses `get_system_prompt(tools_enabled=True, session_id="grok")` (FULL tier)
- Claude: uses `get_system_prompt(tools_enabled=True, session_id="claude")` (FULL tier)
- 9 new tests verify formatted model names, correct tier selection, and session_id formatting

## Tests

1508 passed, 0 failed (41 new tests this cycle)

Fixes #135

Co-authored-by: Kimi Agent <kimi@timmy.local>
Reviewed-on: http://localhost:3000/rockachopa/Timmy-time-dashboard/pulls/138
Reviewed-by: rockachopa <alexpaynex@gmail.com>
Co-authored-by: hermes <hermes@timmy.local>
Co-committed-by: hermes <hermes@timmy.local>

src/timmy/backends.py

@@ -18,7 +18,7 @@ import time
 from dataclasses import dataclass
 from typing import Literal
 
-from timmy.prompts import SYSTEM_PROMPT
+from timmy.prompts import get_system_prompt
 
 logger = logging.getLogger(__name__)
 
@@ -128,7 +128,7 @@ class TimmyAirLLMAgent:
     # ── private helpers ──────────────────────────────────────────────────────
 
     def _build_prompt(self, message: str) -> str:
-        context = SYSTEM_PROMPT + "\n\n"
+        context = get_system_prompt(tools_enabled=False, session_id="airllm") + "\n\n"
         # Include the last 10 turns (5 exchanges) for continuity.
         if self._history:
             context += "\n".join(self._history[-10:]) + "\n\n"
@@ -388,7 +388,9 @@ class GrokBackend:
 
     def _build_messages(self, message: str) -> list[dict[str, str]]:
         """Build the messages array for the API call."""
-        messages = [{"role": "system", "content": SYSTEM_PROMPT}]
+        messages = [
+            {"role": "system", "content": get_system_prompt(tools_enabled=True, session_id="grok")}
+        ]
         # Include conversation history for context
         messages.extend(self._history[-10:])
         messages.append({"role": "user", "content": message})
@@ -481,7 +483,7 @@ class ClaudeBackend:
             response = client.messages.create(
                 model=self._model,
                 max_tokens=1024,
-                system=SYSTEM_PROMPT,
+                system=get_system_prompt(tools_enabled=True, session_id="claude"),
                 messages=messages,
             )