[FEATURE]: Add and Remove MCP servers from the TUI dialog with config persistence

Feature hasn't been suggested before.

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Describe the enhancement you want to request

#37712 exposed runtime MCP controls (add, remove, connect, disconnect) over HTTP but explicitly left the TUI surface out of scope:
"No TUI surface for add/remove yet; those are for programmatic clients."

This issue adds that missing TUI surface and makes runtime changes durable by persisting them to the project config file.

Related:

#37308 — core runtime MCP controls (merged)
#37712 — HTTP routes for MCP controls (merged)
#36303 — MCP modal exploration (open, broader scope)
#9871 — /reload slash command (open, complements this)

Problem

Adding or removing MCP servers currently requires editing opencode.json/opencode.jsonc by hand and restarting OpenCode. This is disruptive mid-conversation, if you're deep into a session and realize you need a new MCP server (e.g., you just spun up a local tool server, or you want to connect to a remote endpoint), you have to:

• Exit or background OpenCode
• Edit the config file manually
• Restart OpenCode, losing your conversation flow and mental context

The TUI MCP dialog (accessed via /mcps slash command) currently only supports toggling existing servers on/off. There is no way to add a new server or remove a stale one without leaving the TUI.

Proposed Solution

Ctrl+a and Ctrl+d in the /mcps dialog will let users add and remove MCP servers. Add prompts for server type, name, and connection details. Persist changes to the project config file using jsonc-parser to preserve comments and formatting.

Acceptance Criteria

• Ctrl+a in the /mcps dialog prompts for type, name, and connection details, then adds and connects the server immediately
• Ctrl+d removes the selected server and cleans up its connection
• Both operations persist to the project config file without destroying comments or formatting
• The dialog refreshes status after add/remove
• New servers are usable in the current session without restarting OpenCode

These changes are only for TUI