feat(init): deprecate --no-git flag, gate deprecations at v0.10.0 (#2357)

* feat(init): deprecate --no-git flag, gate deprecations at v0.10.0

- Add deprecation warning when --no-git is used on specify init
- Update --ai deprecation gate from 1.0.0 to 0.10.0
- Update test expectation for the new version gate

Closes #2167

* fix: address PR review feedback

- Update --no-git deprecation message to reference existing 'specify extension'
  commands instead of non-existent --extension flag
- Add test_no_git_emits_deprecation_warning CLI test

* fix: strengthen --no-git deprecation test assertions

Add assertions unique to the --no-git message ('will be removed',
'git extension will no longer be enabled by default') to prevent
false positives from the --ai deprecation panel.