feat(mcp): enable/disable configured MCP servers

[localai-bot]

What

Adds the ability to disable a configured MCP server in place, without removing it — the engine-side foundation for a per-server enable/disable toggle in the Dante Desktop UI (mirroring how plugins/skills already toggle), plus the matching CLI verbs.

Config / API

• types.MCPServer.Disabled bool (yaml:"disabled,omitempty") — an absent field reads as enabled, so existing configs and previously-added servers stay on with no migration.
• Configurator.SetMCPServerEnabled(name, enabled) error — toggles the flag in the user config file; errors on unknown name, matching GetMCPServer/RemoveMCPServer.
• MCPServerInfo.Enabled (= !Disabled) — ListMCPServers returns all servers with their state.
• EffectiveConfig() drops disabled servers from .MCPServers, so a disabled server starts no transport, while the listing still shows it.

This cleanly separates "what to show" (ListMCPServers, all servers) from "what to run" (EffectiveConfig, enabled only).

CLI

• nib mcp enable <name> / nib mcp disable <name> — mirrors nib plugin enable|disable / nib skill enable|disable (previously nib mcp had only add|list|remove|test).
• nib mcp list appends (disabled) to disabled servers; enabled-server output is unchanged.

Testing

• go test ./..., go build ./..., go vet ./..., gofmt — green.
• Unit tests: SetMCPServerEnabled toggle lifecycle + unknown-name error + listing projection; enabledMCPServers drop/keep/no-mutation/nil→nil; CLI mcp enable|disable exit codes + list disabled marker.
• Verified round-trip: disable → persist YAML → config.Load reload → excluded from EffectiveConfig; re-enable writes no disabled: key (omitempty).

Integrator note (nib-desktop)

EffectiveConfig reads config.Load() (from config.WritablePath()), so SetMCPServerEnabled's write is reflected only when the Configurator is built with New(base, config.WritablePath()) — the same pre-existing seam as AddMCPServer/RemoveMCPServer, already how the desktop constructs it.

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