feat: add --version flag

[stephantul]

This PR adds a little version flag that can be used to check whether your semble is up to date.

semble --version
semble -V

It just prints __version__, so not a lot going on. In the future, we can enrich this using other metadata (builds etc.)

[greptile-apps]

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

_{Reviews (1): Last reviewed commit: "feat: add --version flag" | Re-trigger Greptile}

[Pringled]

LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/semble/cli.py	100.00% <100.00%> (ø)

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