feat(connection): RFC 8628 device flow + machine credential store + zero-button boot

Summary

Wire the real ai-game.dev sign-in for the Unity Editor plugin (mcp-authorize d1, PR 2 of the chain). Foundation PR #875 already adopted McpPlugin 7.0.0-preview.1 (vendored) and migrated UnityConnectionConfig.Token → CredentialProvider (behavior-preserving). This PR implements the RFC 8628 device-authorization grant against the live authorization server, persists the credential into the shared per-machine credential store, and connects signed-in on boot with zero UI interaction.

Context

• Design refs: .claude/design/mcp-authorize/{03-auth-flows.md (Flow B), 06-engine-plugins.md, 09-user-workflows.md}; task tasks/d1-unity-plugin.md.
• The Authorization Server is live at https://ai-game.dev (issuer).
• McpPlugin 7.0 already ships MachineCredentialStore + MachineCredentials + PluginCredentialProvider (auto-adopt, proactive/reactive refresh via an injected ITokenRefresher) + ConnectionCredentialCoordinator (on-401 refresh→reconnect); this PR provides the Unity device-flow + refresher and wires the provider into the Cloud-mode connection.
• Mirrors the sibling engine device-flow PRs (Godot #265 / Unreal #212).

Proposed approach

1. DeviceAuthService: RFC 8628 alias — POST /oauth/device_authorization (form client_id=unity-mcp-plugin + scope=mcp:plugin) → poll POST /oauth/token with the urn:ietf:params:oauth:grant-type:device_code grant → ES256 access JWT + rotating refresh token.

• 2. DeviceAuthFlow: injectable state machine (client / browser-open / poll-delay injectable) so it runs against a mocked AS with no live network in CI; honours interval / slow_down / authorization_pending / access_denied / expired_token.
• 3. UnityTokenRefresher (ITokenRefresher): exchanges the refresh token for a fresh access token at /oauth/token; fails closed.
• 4. AccountCredentialService: owns a PluginCredentialProvider over the shared machine store (~/.ai-game-dev/credentials.json, 0600 / DPAPI); auto-adopt on boot, proactive refresh at connect, on-401 refresh+reconnect via ConnectionCredentialCoordinator.
• 5. Wire the Cloud credential provider into UnityConnectionConfig.CredentialProvider (machine-store JWT wins in Cloud mode; falls back to the mode-routed token; ignored in Custom/Local).

Acceptance criteria

• Zero-button: a seeded machine store → editor boots connected in Cloud mode with no UI interaction.
• Device-flow / store / refresher / provider unit tests pass against a mocked authorization server (no live network in CI).
• Unity plugin compiles; full EditMode suite green on the local worktree Editor.
• Unity CI matrix green.

Scope note: instance-metadata handshake / ProjectIdentity marker (PR 3), port change (PR 4), config-writer UI flip / token-field removal (PR 5), and live e2e (PR 6) are out of scope for this PR.

Start with tasks/d1-unity-plugin.md and the referenced Flow B design, then inspect UnityConnectionConfig.CredentialProvider and the sibling Godot and Unreal device-flow changes. Run the mocked authorization-server unit tests and the Unity EditMode suite. Done means the seeded machine store enables zero-button Cloud startup, device flow and refresh behavior pass without live network access, and the Unity CI matrix is green.