Add notice that git extension will no longer be enabled by default at 1.0.0

[mnriem]

Summary

The git extension is currently bundled and enabled by default during specify init. Starting with v1.0.0, it will still be bundled but no longer enabled by default — users will need to explicitly opt in. We should add messaging now so users are aware of the upcoming change.

Proposed behavior

During specify init, display a notice when the git extension is auto-enabled, e.g.:

⚠️ The git extension is currently enabled by default, but starting with v1.0.0 it will require explicit opt-in via specify init --extension git or post-init with specify extension add git.

Scope

• Notice should appear only during specify init
• No changes to the git extension itself or its bundled status
• The git extension remains bundled (shipped with spec-kit) — only the default-enabled behavior changes at 1.0.0

Acceptance criteria

• specify init shows a notice when the git extension is auto-enabled
• Notice is visible but non-blocking (yellow/warning styling)
• Message clearly states the timeline (v1.0.0) and how to opt in