🤖 feat: improve MCP servers (global config, status, startup)

[ibetitsmike]

Summary

Implements MCP improvements from issue #2060:

• Adds a global/shared MCP config file (~/.mux/mcp.jsonc) and merges global + project servers.
• Makes MCP server startup resilient (parallel startup + per-server timeouts) so one hung server can’t block a send.
• Exposes per-server runtime status + restart controls in the Workspace MCP modal.
• Emits a non-spam chat warning when MCP tools are unavailable due to startup failures.

Background

Issue: #2060

Implementation

• Config scopes
  • Global config: ~/.mux/mcp.jsonc
  • Project config: <project>/.mux/mcp.jsonc
  • Precedence: global < project < inline
• Startup reliability
  • Adds timeouts around spawn/connect/list-tools and starts servers in parallel.
• Workspace UX
  • Workspace MCP modal now shows merged servers, origin (global/project), runtime status, and restart buttons.
  • Restart is disabled while a stream is active or when there are unsaved overrides.
• Chat warnings
  • Injects a synthetic assistant warning message on send when MCP servers fail, with dedupe/cooldown.

Validation

• make static-check

Risks

• Servers that take >10s to spawn/connect/list tools will be treated as failed and retried later; this is intentional to avoid hangs.

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