feat: add plugin commands

[MarioCadenas]

Introduces a full suite of CLI commands under npx appkit plugin for managing plugin manifests — creating, validating, listing, syncing, and adding resources. Restructures the previous single plugins sync command into a modular command tree and strengthens the JSON schema with type-specific permission validation.

New CLI commands

• plugin create — Interactive wizard (via @clack/prompts) to scaffold a new plugin with manifest, TypeScript class, and barrel exports. Supports in-repo or standalone placement, resource selection, and metadata fields.
• plugin validate — Validates manifest.json or appkit.plugins.json against the JSON schema. Auto-detects schema type, reports humanized errors with actual values and expected enums.
• plugin list — Lists plugins from appkit.plugins.json or scans a directory. Supports --json output for scripting.
• plugin add-resource — Interactively adds a resource requirement to an existing plugin manifest.
• plugin sync — Refactored from the previous plugins sync; discovers plugin manifests from packages and local imports, writes consolidated appkit.plugins.json.

Schema improvements

• Type-specific permission validation: Each resource type (12 known types) now has its own permission enum enforced via allOf/if-then rules (e.g. sql_warehouse only allows CAN_MANAGE | CAN_USE).
• No unknown resource types: The resourceType enum is the single source of truth — unknown types are rejected at the schema level.
• Template schema deduplication: template-plugins.schema.json now $refs definitions from plugin-manifest.schema.json instead of duplicating them.