v0.9.0 — GitHub App connection for hosted users

Added

• Hosted mode: GitHub connection through a GitHub App (SEO_MCP_GITHUB_APP_*). Users install the app on the repositories they choose; the server verifies the installation against the signed-in GitHub user (OAuth code returned with the installation), stores only the installation id and mints one-hour installation tokens on demand. Connected users get the github_* toolset including github_commit_files and github_commit_image while the rest of their server stays read-only; Disconnect uninstalls the app. GitHub tools inside a hosted request never fall back to the operator's GITHUB_TOKEN.
• ServerOptions.allowWrite (write-tool prefixes that survive readOnly); write-audit lines carry who for hosted users; google_auth_status reports the scopes actually granted by the user.