Add legacy migration guides and cleanup tools

[masahide]

Summary

• add a prominent README notice for users upgrading from legacy OmniSSHAgent
• add the redesign background and a complete migration guide
• add a read-only Windows diagnostic for legacy settings, key references, credentials, services, autostart, sockets, and optional WSL inspection
• add a curl-executable WSL2 legacy proxy uninstaller with --dry-run, exact executable matching, process shutdown, profile backups, and path-restricted cleanup
• move planning and requirements documents under docs/
• remove the obsolete examples/ tray sample
• keep signing notifications as a reserved future feature and remove the non-functional tray toggle

WSL2 uninstall

Preview:

curl -fsSL https://raw.githubusercontent.com/masahide/OmniSSHAgent/main/scripts/uninstall-legacy-wsl2.sh |
  sh -s -- --dry-run

Apply after merge:

curl -fsSL https://raw.githubusercontent.com/masahide/OmniSSHAgent/main/scripts/uninstall-legacy-wsl2.sh |
  sh

The script stops only $HOME/wsl2-ssh-agent-proxy/wsl2-ssh-agent-proxy, waits for TERM before KILL, backs up affected shell profiles, and removes only the dedicated proxy directory and socket.

Validation

• go test ./...
• go vet ./...
• PowerShell 5.1 parse and execution of scripts/check-legacy-install.ps1
• static check that the Windows diagnostic contains no mutating commands
• bash -n scripts/uninstall-legacy-wsl2.sh
• isolated-home --dry-run through Git for Windows Bash
• stale doc/ and deleted examples/ reference checks
• git diff --check