docs: sync Unity/CLI/Installer READMEs with shipped OAuth login, pinned setup-mcp, and current auth vocabulary

[IvanMurzak]

Summary

The public-facing READMEs (root README.md, cli/README.md, and the Installer README.md) drifted from shipped behavior after the CLI OAuth-login migration and the cloudToken cleanup. This updates them to describe the current OAuth device-flow login, path-optional install-plugin, pinned setup-mcp, and the current none / oauth / token auth vocabulary — and regenerates the Installer README from root (it listed 3 extensions instead of 10 and lacked the Uninstall section).

Context (shipped behavior)

• unity-mcp-cli login runs the OAuth 2.1 device flow and stores the credential in the shared machine store (~/.ai-game-dev/credentials.json); there is no PAT to paste.
• install-plugin path is optional (cwd + Packages/manifest.json marker probe); also supports --with-server and --enroll.
• setup-mcp writes a credential-free, project-pinned config by default (/mcp/p/<pin>), with --no-pin to opt out.
• The legacy required auth mode was removed; plugin auth modes are none / oauth / token (a persisted required migrates to token).

Proposed approach

• R1 README.md: login steps → OAuth wording; server + plugin auth vocab → none / oauth / token; fix the CI batch example (UNITY_MCP_CLOUD_URL, AUTH_OPTION=token); add pinned-URL / enroll coverage.
• R2 cli/README.md: document login in the command reference; make install-plugin path optional (+ enroll/server options); open --url → UNITY_MCP_CLOUD_URL; document pinned setup-mcp (--no-pin).
• R3 Installer/.../README.md: regenerate from root (10 extensions + Uninstall section) + the same auth fixes.
• No legacy device-flow login documented anywhere.

Acceptance criteria

• All listed line-item fixes applied; commands match shipped CLI behavior.
• Installer README regenerated with the current extension list + Uninstall section.
• No legacy-login documented (grep).