feat(plugins): add CLI enable/disable for plugin.json

[Ayush7614]

Summary

Add zero plugins enable <id> and zero plugins disable <id> so installed plugins can be toggled via the enabled field in plugin.json without removing them.

Motivation

Hooks and MCP already support enable/disable. Plugins currently require remove/re-add (or hand-editing plugin.json) to deactivate. EXTENDING.md previously called this out as a gap.

Proposed behavior

• zero plugins enable <id> / zero plugins disable <id>
• Flags: --user (user plugins only), --json
• Idempotent: already-enabled / already-disabled reports a clear no-op message
• Project plugins take precedence over user plugins unless --user is set
• Preserve existing plugin.json file mode when rewriting

Related PR

• feat(plugins): add CLI enable/disable for plugin.json #693

Notes for maintainers

Please add the issue-approved label if this scope is accepted so the linked community PR can proceed under CONTRIBUTING.md.