Add __version__ attribute to sgraph package

[villelaitila]

Summary

Expose the standard sgraph.__version__ attribute, resolved at import time from installed package metadata via importlib.metadata.version().

• Single source of truth: the version comes from package metadata (setup.cfg), nothing to keep in sync manually.
• Falls back to "unknown" when the package is not installed (e.g. running from a plain source checkout), so importing never breaks.
• Useful for bug reports, runtime diagnostics, and downstream consumers such as sgraph-mcp-server that want to report or check the library version.

Originally authored on the supporting-__version__ branch (Feb 2026); rebased onto current main with a test added.

Test plan

• New test test_version_attribute asserts the attribute exists and is a non-empty string (passes both installed and uninstalled thanks to the fallback).
• Full suite: 162 passed.

🤖 Generated with Claude Code

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Softagram Impact Report for pull/163 (head commit: 4444b07)

TL;DR Changed code files: 2 | Directly impacted code files: 0

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