Ollama: pass discovered num_ctx in chat requests

_build_ollama_payload sends options.temperature and options.num_predict
to /api/chat, but never options.num_ctx. Ollama defaults num_ctx to 2048
when the option is omitted, so prompts going to any Ollama backend are
silently truncated there regardless of the model's actual capability.

Thread the discovered context length through the three call sites
(llm_call, llm_call_async, stream_llm) and emit options.num_ctx when it
is known and positive. The builder filters out the DEFAULT_CONTEXT
fallback (128000) so we don't lie to Ollama about models whose window
we couldn't actually discover. The issue's literal 'when > 2048'
heuristic is dropped: a model with a real context smaller than 2048
would OOM if Ollama used its default, so we pass the real value
regardless of size. Matches how src/context_compactor.py uses the
same helper.

Sister fix to PR #753 — that PR teaches the compactor the right budget,
this one tells Ollama to actually use that budget on the way in.

src/llm_core.py

@@ -8,6 +8,7 @@ import hashlib
 import threading
 from fastapi import HTTPException
 from typing import Optional, Dict, List
+from src.model_context import get_context_length, DEFAULT_CONTEXT
 from urllib.parse import urlparse
 
 logger = logging.getLogger(__name__)
@@ -238,7 +239,19 @@ def _build_ollama_payload(
     max_tokens: int,
     stream: bool = False,
     tools: Optional[List[Dict]] = None,
+    num_ctx: Optional[int] = None,
 ) -> Dict:
+    """Build the JSON payload for Ollama's /api/chat endpoint.
+
+    ``num_ctx`` sets the input context window. Ollama defaults to 2048
+    when the option is omitted, so a model with a larger advertised
+    window is silently truncated there, and a model with a smaller one
+    gets an oversized window it can't service. Pass the discovered
+    context length through ``num_ctx``; this builder only emits it when
+    the value is trusted (not the ``DEFAULT_CONTEXT`` fallback), so we
+    don't guess for unknown models but do tell Ollama the real window
+    when we know it — even if it's smaller than 2048.
+    """
     payload: Dict = {
         "model": model,
         "messages": _ollama_normalize_tool_messages(messages),
@@ -249,6 +262,8 @@ def _build_ollama_payload(
         options["temperature"] = temperature
     if max_tokens and max_tokens > 0:
         options["num_predict"] = max_tokens
+    if num_ctx is not None and num_ctx > 0 and num_ctx != DEFAULT_CONTEXT:
+        options["num_ctx"] = num_ctx
     if options:
         payload["options"] = options
     if tools:
@@ -675,7 +690,10 @@ def llm_call(url: str, model: str, messages: List[Dict], temperature: float = LL
         payload = _build_anthropic_payload(model, messages_copy, temperature, max_tokens)
     elif provider == "ollama":
         target_url = _normalize_ollama_url(url)
-        payload = _build_ollama_payload(model, messages_copy, temperature, max_tokens, stream=False)
+        payload = _build_ollama_payload(
+            model, messages_copy, temperature, max_tokens,
+            stream=False, num_ctx=get_context_length(url, model),
+        )
     else:
         target_url = url
         payload = {
@@ -790,7 +808,10 @@ async def llm_call_async(
         h = {"Content-Type": "application/json"}
         if headers:
             h.update(headers)
-        payload = _build_ollama_payload(model, messages_copy, temperature, max_tokens, stream=False)
+        payload = _build_ollama_payload(
+            model, messages_copy, temperature, max_tokens,
+            stream=False, num_ctx=get_context_length(url, model),
+        )
     else:
         target_url = url
         h = _provider_headers(provider, headers)
@@ -888,7 +909,10 @@ async def stream_llm(url: str, model: str, messages: List[Dict], temperature: fl
         h = {"Content-Type": "application/json"}
         if headers:
             h.update(headers)
-        payload = _build_ollama_payload(model, messages_copy, temperature, max_tokens, stream=True, tools=tools)
+        payload = _build_ollama_payload(
+            model, messages_copy, temperature, max_tokens,
+            stream=True, tools=tools, num_ctx=get_context_length(url, model),
+        )
     else:
         target_url = url
         payload = {