fix(devin): connect T3 tools for new and resumed sessions

[ProtonDev-sys]

Devin can chat through ACP but ignores session/new.mcpServers, so T3's preview tools never reach the agent. The runtime also constructed additionalDirectories that the generated RPC encoder silently dropped.

Connect T3's tools through Devin's MCP extension using a private configuration directory owned by the provider session. Preserve additional workspace roots when creating, loading, and resuming sessions, and remove the temporary credentials when the session closes. This reuses the existing ACP request and HTTP paths.

This is a focused follow-up to the existing Devin integration, targeting its feature branch rather than duplicating the provider implementation.

Validation:

• 95 focused tests passed across the Devin adapter/provider/configuration and ACP client/agent/protocol suites.
• Server and effect-acp type checks, lint on changed files, and the production web/server build passed.
• Devin CLI 3000.10.21 on Windows: real preview tool calls passed with Adaptive and SWE-1.7; the extended smoke test resumed a session with a fresh credential, called the tool again, and read/wrote project files.
• Built-app browser check: selected Devin's SWE-1.7 Lightning model, sent a request through the composer, and verified the resulting file and assistant reply.

Model and harness: GPT-6 in Codex.