Make git extension opt-in and remove `--no-git` at 0.10.0

[mnriem]

Summary

At 0.10.0, finalize the git extension default change and --no-git removal:

1. Git extension becomes opt-in — no longer auto-enabled during specify init. Users must pass --extension git or add it post-init with specify extension add git.
2. Remove --no-git flag — no longer needed since the git extension won't be enabled by default.

This is the follow-through on the deprecation notices added earlier.

Depends on

• Add notice that git extension will no longer be enabled by default at 0.10.0 #2165 (deprecation notice for git extension default)
• Add --extension flag to specify init for opting into extensions at init time #2166 (--extension flag on specify init)
• Deprecate --no-git flag on specify init (removal at 0.10.0) #2167 (deprecation notice for --no-git)

Acceptance criteria

• specify init no longer auto-enables the git extension
• Git extension remains bundled (shipped with spec-kit) and installable via --extension git
• --no-git flag is removed
• Documentation updated to reflect the change
• CHANGELOG entry noting the breaking change