feat: emit init-time notice for git extension default change

[mnriem]

Summary

Add a non-blocking Panel notice during specify init when the git extension auto-enables, informing users that starting in v0.10.0 this will require explicit opt-in via specify extension add git.

Changes

File	Change
src/specify_cli/__init__.py	Track successful git extension install; display yellow "Notice: Git Default Changing" panel after init completes
tests/integrations/test_cli.py	Integration test validating notice content (v0.10.0 timeline, opt-in messaging, migration command)
docs/reference/core.md	User-facing NOTE about the upcoming default change

User-facing behavior

After specify init completes (when the git extension auto-installs), users see:

╭──────────────────────── Notice: Git Default Changing ────────────────────────╮
│                                                                              │
│  The git extension is currently enabled by default during specify init.      │
│  Starting in v0.10.0, this will require explicit opt-in.                     │
│  Use specify extension add git after init when needed.                       │
│                                                                              │
╰──────────────────────────────────────────────────────────────────────────────╯

Related

• Closes Add notice that git extension will no longer be enabled by default at 0.10.0 #2165
• Related: Deprecate --no-git flag on specify init (removal at 0.10.0) #2167 (deprecate --no-git, merged), Make git extension opt-in and remove --no-git at 0.10.0 #2168 (removal at 0.10.0, open)

[Copilot]

Pull request overview

Adds a user-facing, non-blocking warning during specify init to inform users that the bundled git extension’s default auto-enable behavior will change in v0.10.0 (requiring explicit opt-in), with corresponding test coverage and docs.

Changes:

• Track successful bundled git extension installation during specify init and emit a yellow Rich Panel notice after init completes.
• Add an integration test asserting the notice includes the v0.10.0 timeline and the opt-in/migration command.
• Document the upcoming default change in the core command reference.

Show a summary per file

File	Description
src/specify_cli/__init__.py	Sets a flag when the bundled git extension installs successfully and prints a post-init notice panel.
tests/integrations/test_cli.py	Adds an integration test validating the notice text components in normalized CLI output.
docs/reference/core.md	Adds a NOTE admonition about the upcoming opt-in requirement and the post-init command to add git.

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• Files reviewed: 3/3 changed files
• Comments generated: 0