Finalize v1.0 user documentation and troubleshooting guide

[devgamesan]

Context

README.md, README.ja.md, docs/manual-testing.md, docs/troubleshooting.md, and docs/spec.md are already substantial. Before v1.0, they should be reviewed as a coherent user-facing documentation set.

Tasks

• Ensure README.md and README.ja.md describe the same supported workflows.
• Verify quick start commands match current CLI behavior.
• Confirm troubleshooting covers common failures: SSH auth, Git auth, dirty worktree, diverged branches, gitsync remote mismatch, LFS/submodules, Windows path quoting.
• Add a concise v1.0 release checklist for maintainers.
• Remove or mark stale content that no longer reflects current behavior.

Acceptance Criteria

• A new user can follow the quick start without relying on undocumented assumptions.
• English and Japanese docs are consistent on support scope and limitations.
• Troubleshooting links are discoverable from README.
• Maintainer release steps are documented.