fix(tui): persist plugin activation choices

[kitlangton]

Problem

In the V2 TUI, /plugins currently mutates only the running registration. Editing any plugin source triggers reconciliation from config, which can re-enable unrelated plugins that the user manually disabled. The choice also does not survive a TUI restart.

Desired Behavior

A plugin disabled through /plugins remains disabled when:

• another plugin source reloads;
• its own source reloads;
• the TUI restarts.

Enabling it again should remove or replace that durable preference without disturbing its package declaration or options.

Design Question

Choose a durable config representation. The simplest candidate for declared external plugins is:

{
  "package": "/path/to/plugin.ts",
  "enabled": false,
  "options": {}
}

Built-ins and auto-discovered plugins still require an ID-based override or another explicit representation because they have no package declaration.

PR #42410 demonstrates the bug, regression coverage, and an initial ordered-directive implementation. Finalize the config model before merging.

Acceptance Criteria

• Activation choice is durable and config-backed.
• Unrelated hot reload does not reactivate disabled plugins.
• Package paths and options remain intact.
• Built-in and discovered plugin behavior is explicit.
• Reload and restart behavior have regression coverage.