Deprecate `--no-git` flag on `specify init` (removal at 0.10.0)

[mnriem]

Summary

The --no-git flag on specify init skips git repository initialization and git extension installation. With the introduction of --extension (#2166) and the git extension no longer being enabled by default at 0.10.0, --no-git becomes redundant and should be deprecated now and removed at 0.10.0.

Proposed behavior

• Add a deprecation warning when --no-git is used: "⚠️ --no-git is deprecated and will be removed in v0.10.0. The git extension will no longer be enabled by default — use --extension git to opt in."
• --no-git continues to function until 0.10.0
• Removal happens in #TBD (0.10.0 issue)

Depends on

• Add --extension flag to specify init for opting into extensions at init time #2166 (--extension flag)

Acceptance criteria

• Using --no-git emits a visible deprecation warning
• --no-git still functions as before (non-breaking)
• Remove --no-git at 0.10.0