Environment-level MCP servers (and later rules/skills) shared by every provider and account

[x56sandmanx]

Problem
When you run more than one provider account in T3 Code, each account keeps its own MCP config. Adding a server like Linear means setting it up again per account. Rules and skills have the same problem.

Idea
Let T3 Code own a list of MCP servers per environment and attach them to every provider session, next to the t3-code server it already injects. Every adapter already has a per-session MCP hook, so this extends a list that already exists rather than adding new machinery:

• Claude: SDK mcpServers
• Codex: -c mcp_servers.* overrides
• Cursor, Grok and Antigravity: ACP session/new
• OpenCode: mcp.add

Details:

• Where: Settings → Providers → MCP servers. Command (stdio) or URL (streamable HTTP) servers, an on/off switch, and all projects or specific projects.
• Secrets: sensitive env vars and headers use the existing secret-store and redaction path, like provider environment secrets.
• Remote: servers resolve on the environment that runs the agent, so it behaves the same over LAN, Tailscale and Connect.
• Not included yet: OAuth. Servers that need a browser login work through mcp-remote or a bearer token.

Status
I have a working prototype on a local branch that I use on my own build. It's about 1.3k lines including tests and a user doc, so I'm not opening a PR. If this direction fits, I'm happy to split it into small PRs (contract + settings, server injection, settings UI), or you can build it your own way.

Rules and skills would be natural follow-ups. Rules can reuse the existing runtime-instructions append. Skills are harder, because each CLI only finds them by scanning its own folders.