Add model calibration and uncertainty notes

[sardorcodev]

Document model probability limitations and future calibration work.

Scope:

• Explain that current confidence/probability values are model scores, not field-validated certainty.
• Add calibration as a future ML reliability task.
• Update MODEL_CARD.md and MODEL_RELEASE_NOTES.md.

Acceptance criteria:

• Model probability limitations are documented.
• No production agronomic reliability is claimed.
• Calibration is listed as a future task.