refactor: derive package version from metadata

[StatPan]

Summary

• remove the hard-coded assemblymcp.__version__ constant
• derive package version from installed metadata instead
• add a regression test to keep runtime version and package metadata aligned

Why

• automatic version bumps were updating pyproject.toml without keeping assemblymcp.__version__ in sync
• this makes runtime version reporting depend on a single source of truth

Validation

• uv run ruff check .
• uv run ruff format --check .
• uv run pytest -q -> 68 passed

[gemini-code-assist]

Code Review

This pull request updates the version of the assemblymcp package to 0.6.4 in both the source code and the lockfile. I have no feedback to provide.

[StatPan]

Merged manually into master after repeated GitHub merge API failures (502 / merge already in progress). The merged content is identical to this PR branch. Local verification after merge: uv run pytest -q -> 68 passed.

[StatPan]

Closing this PR because its changes have been merged manually into master as commit 8b193e3 after GitHub merge API instability. The branch content is now on master.