Replace outdated v0.1 wording before v1.0

[devgamesan]

Context

Several user-facing messages and docs still say v0.1, including LFS/submodule warnings and docs/spec.md. This will be confusing for a v1.0 release.

Known locations include:

• docs/spec.md
• src/git_ssh_sync/status.py
• src/git_ssh_sync/doctor.py
• tests that assert the old wording

Tasks

• Replace user-facing v0.1 wording with version-neutral language or v1.0-accurate wording.
• Update tests that assert the current messages.
• Decide whether docs/spec.md remains a historical v0.1 spec or should become a current product spec.
• Ensure README/README.ja limitations match the final wording.

Acceptance Criteria

• No user-facing v0.1 wording remains unless clearly marked as historical.
• LFS and submodule limitations are documented consistently.
• Tests pass after message updates.