Move from agent-specific to generic context consumer API
## Problem/motivation
CCC was initially focused on agents, but other consumers will also want context.
Example:
- ai_ckeditor - #3581955+s
- automators - #3563106+s
- other contributed and custom modules
## Solution
Rearchitect the code to center on context consumers, with agents as just one type of consumer.
Fable 5 plan from:
#3586318+s
[ai_context-consumer-rearchitecture-plan-v3.md](/uploads/43d12b8f77e375c268ef3f9dc0b6bd40/ai_context-consumer-rearchitecture-plan-v3.md)
## AI usage
- [x] AI assisted issue
issue