[Plugins] Enable, disable, and switch active plugin profiles #1071

Part of #1068.

Scope

Allow consumers to enable, disable, and switch between installed plugins - including switching between alternative implementations of the same role (e.g., two memory plugins).

Requirements

• squad plugin enable <name> / squad plugin disable <name> toggle a plugin's active state without uninstalling it.
• squad plugin switch <role> <name> makes <name> the active plugin for <role> (e.g., memory, knowledge).
• Active state is persisted (e.g., .squad/plugins/active.json) and is the single source of truth consulted by Squad at runtime.
• Disabling a plugin must not delete its files; re-enabling restores prior behavior.
• squad plugin list shows installed plugins, their version, enabled/disabled state, and which role(s) each is active for.
• Switching is observable: a downstream test can assert that switching changes the agent-visible behavior (paired with the demo/test child issue).

Acceptance

• Enable/disable/switch commands implemented with persisted state.
• list output is stable and machine-readable (JSON flag).
• Tests cover enable->disable->re-enable and role switching.

Planning provenance: tamirdresher_microsoft/tamresearch1#2919.