Support manual connect for configured MCP servers

[SivanCola]

Summary

Adds explicit MCP auto-start control and a manual connect path:

• [[plugins]].auto_start = false keeps a configured MCP server from connecting during session startup.
• /mcp connect <name> connects a configured but currently disconnected server on demand.
• reasonix mcp list shows [auto_start=false] for disabled startup entries.
• Chat completion offers /mcp connect and suggests disconnected configured server names.
• .mcp.json imports preserve auto_start when present.

Root Cause

Some MCP servers are useful but heavy, flaky, auth-bound, or only needed occasionally. Starting every configured server on every session makes startup slower and can introduce unnecessary provider-visible tool schema churn.

Technical Approach

• Adds PluginEntry.AutoStart *bool so omitted values preserve historical auto-start behavior.
• Adds PluginEntry.ShouldAutoStart() and Config.AutoStartPlugins().
• Filters startup MCP specs in normal boot and ACP session boot through AutoStartPlugins().
• Adds controller helpers for configured/disconnected MCP names and ConnectConfiguredMCPServer.
• Extends /mcp connect chat handling and structured slash completion.
• Extends TOML render/round-trip tests and .mcp.json parsing tests for auto_start=false.

Focused Optimization Points

• auto_start is tri-state by using *bool: nil keeps current behavior, false opts out, true can be rendered/preserved.
• Manual connect is an explicit user action; there is no automatic retry loop or per-turn dynamic reordering.
• Configured plugin order remains unchanged.
• No v1 frontend, attachment, autoscroll, or startup-resilience changes are included.

Cache / Tool Schema Notes

This PR intentionally lets users keep configured MCP servers out of the startup tool schema with auto_start=false. That can improve cache stability for heavy or rarely used servers, but connecting one manually changes the provider-visible tool set on the next model turn, matching existing live /mcp add behavior.

Verification

Passed:

• go test ./internal/config ./internal/control ./internal/cli ./internal/acp ./internal/boot -run 'TestPlugin|TestAutoStart|TestMCP|TestSlashArg|TestBuildWithoutMemoryLeavesPromptUnchanged|TestRenderTOML|TestLoadMCP|TestMergeMCP|TestProtocol'
• go test ./internal/agent -run TestPlanModeDoesNotMutateSystemOrTools

[chatgpt-codex-connector]

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