nuextract3: add --instructions for chat_template_kwargs.instructions

Exposes NuExtract3's `instructions` chat-template kwarg, a free-text
steering lever for output formats and routing across union/optional
schema branches. Mirrors the existing `--enable-thinking` passthrough:
new arg, plumbed via main() into chat_template_kwargs only when truthy
(no-op when omitted).

Docs: https://huggingface.co/numind/NuExtract3#instructions

Smoke (davanstrien/ufo-ColPali, 3 samples, extraction mode,
template={"title": "string", "author": "string"},
instruction="Prepend 'INSTR-OK::' to the title value"):
sentinel landed in 3/3 extractions.

Teklia French vital-records (58 cards, superset Baptême/Mariage/Décès
template + type_acte enum + French routing instruction): strict
F1 0.281 -> 0.321. Modest; the model partially honours the
branch-null routing (emits empty {} subdicts rather than null for
non-matched branches), which is consistent with NuExtract3's
shape-binding template engine. Per-user instruction tuning lives
downstream of this flag.

nuextract3.py

@@ -27,6 +27,10 @@ Modes are selected via flags:
 - --template SOURCE  -> structured extraction with a NuExtract template
 - --schema SOURCE    -> structured extraction with a JSON Schema
                         (auto-converted via numind.nuextract_utils)
+- --instructions STR -> free-text guidance passed through to the model
+                        (output-format rules, branch routing, etc.).
+                        Combines with any of the modes above.
+                        See https://huggingface.co/numind/NuExtract3#instructions
 
 --template / --schema each accept inline JSON, a URL, or a local file path, so a
 schema can be hosted (e.g. on an HF dataset's raw URL) and reused across jobs:
@@ -277,6 +281,7 @@ def main(
     template_arg: Optional[str] = None,
     schema_arg: Optional[str] = None,
     enable_thinking: bool = False,
+    instructions: Optional[str] = None,
     temperature: Optional[float] = None,
     model: str = MODEL_DEFAULT,
     hf_token: str = None,
@@ -351,6 +356,8 @@ def main(
         chat_template_kwargs["template"] = json.dumps(template, indent=4)
     else:
         chat_template_kwargs["mode"] = mode
+    if instructions:
+        chat_template_kwargs["instructions"] = instructions
 
     logger.info(f"Processing {len(dataset)} images in batches of {batch_size}")
     logger.info(f"Output will be written to column: {output_column}")
@@ -646,6 +653,16 @@ Examples:
         action="store_true",
         help="Enable reasoning mode (slower, better on hard documents)",
     )
+    parser.add_argument(
+        "--instructions",
+        default=None,
+        help=(
+            "Free-text instructions passed to NuExtract via "
+            "chat_template_kwargs.instructions (e.g. routing guidance across "
+            "optional schema branches, output-format rules). "
+            "See https://huggingface.co/numind/NuExtract3#instructions"
+        ),
+    )
     parser.add_argument(
         "--temperature",
         type=float,
@@ -712,6 +729,7 @@ Examples:
         template_arg=args.template,
         schema_arg=args.schema,
         enable_thinking=args.enable_thinking,
+        instructions=args.instructions,
         temperature=args.temperature,
         model=args.model,
         hf_token=args.hf_token,