fix: carry the agent's system facts into agent.extract

[kevin9327]

Problem

agent.extract(...) drops the system facts the agent was built with.

def extract(self, text, schema, max_new_tokens=256, strict=True):
    return extract(text, schema, max_new_tokens=max_new_tokens,
                   weights=self._weights, strict=strict)

weights and strict are forwarded; self._system is not, so the module-level extract builds its
one-shot agent with system=None.

That matters because system facts are what license relative language. From the docs:

Relative language ("tomorrow at 7") resolves only when a date: fact licenses it.

So an agent created the documented way:

agent = needle.Needle(tools=tools, system="date: 2026-07-21 Tue 14:30; locale: en-US")
agent.extract("dinner tomorrow at 7", Reservation)

extracts without the date it was given, while the module-level call one line away keeps it:

needle.extract("dinner tomorrow at 7", Reservation, system="date: 2026-07-21 Tue 14:30")

With strict=True, the default, the loss compounds: _temporal_grounding checks extracted dates
against years found in the source, and an unlicensed relative date has nothing to ground against.

Observed on main by watching what the inner agent is constructed with:

system the agent was built with : 'date: 2026-07-21 Tue 14:30; locale: en-US'
system reaching extract's Needle: None

Fix

Forward self._system, decoded, with an empty string sent as None so an agent built without facts
behaves exactly as before.

Tests

Three in tests/test_weights.py, on the existing engine fixture:

• test_agent_extract_carries_its_own_system_facts — the facts reach extract. Fails before.
• test_agent_extract_without_system_facts_sends_none — no facts still means None, not "".
• test_agent_extract_still_carries_weights_and_strict — the two arguments that already worked keep
  working, alongside the facts. Fails before.

How I tested

Windows 11, Python 3.12.10. pytest -q -m "not slow" gives 67 passed, 9 skipped, up from 64 by the
three new tests; nothing else moves. The 9 skips are the six engine tests and the three worker tests
that want a C compiler.

test_lora.py, test_render.py, test_run.py, test_build.py and test_finetune.py are excluded:
they need flax, whose orbax-checkpoint dependency will not install on Windows because its paths
exceed MAX_PATH. Nothing here touches them, and CI runs them on Ubuntu.

One thing worth a maintainer's opinion: Needle.extract is public but is not in llms.txt or
doc/apis.md, which document only the module-level extract. Happy to add a line for it in either
file if you want it documented rather than quietly available.