Migrate hook system dev docs to public docs site

[ajbozarth]

docs/dev/hook_system_implementation_plan.md is a stale implementation plan (the implementation is shipped). The live reference for the hook system is docs/dev/hook_system.md, which already points to concepts/plugins as the intended public page — but that page doesn't exist yet and is a broken nav entry in docs.json.

Tasks:

• Write docs/docs/concepts/plugins.md covering plugin usage, registration, execution modes, hook types reference, and common patterns
• Remove or archive docs/dev/hook_system_implementation_plan.md
• Verify docs/dev/hook_system.md internal rationale is still accurate or update as needed

[ajbozarth]

Note: when writing the concepts/plugins page, make sure to include GENERATION_ERROR in the hook types reference (added in #856).