Fix git describe abbrev length for reproducibility

[sbellem]

git describe --long abbreviation length

This PR adds a GIT_ABBREV_LEN constant (set to 9) and uses --abbrev=9 to ensure consistent version strings regardless of clone depth.

The abbreviated hash length varies based on the number of objects in the local repository.

Git picks the shortest abbreviation that's unique among all objects it knows about:

• Shallow clone → fewer objects → shorter hash (often 7 chars)
• Full clone → more objects → longer hash may be needed (8, 9, 10+ chars)

Two builds of the same commit can produce different version strings:

Environment	Output
Shallow CI clone	v0.10.0-20-ga8eca71
Full local clone	v0.10.0-20-ga8eca714d

Both are valid but different, breaking reproducibility.

[bunnie]

good catch. thanks for fixing this!