Add agent/system_prompts/*.txt to package-data

[yama-is-bocchi]

Description

This PR fixes an issue where installing sam3 via uv add or a Git-based
dependency (as shown below) causes a FileNotFoundError for the system prompt.

[project]
dependencies = [
    "sam3 @ git+https://github.com/facebookresearch/sam3.git@5eb25fb54b70ec0cb16f949289053be091e16705",
]

How the issue was discovered

The error occurs when running agent_inference in sam3/agent/agent_core.py
without cloning the repository (i.e., when sam3 is installed as a dependency).

Specifically, a FileNotFoundError is raised at the following line, where the
system prompt file is expected to exist relative to the repository structure:

sam3/sam3/agent/agent_core.py

Line 174 in 5eb25fb

with open(MLLM_SYSTEM_PROMPT_PATH, "r") as f: