hooks: resolve UserInstructions provider precedence

[anp-oai]

why

Multiple discovered providers need deterministic precedence, and managed requirements must not silently fall back when their selected provider is unavailable.

what

• selects the highest-priority active UserInstructions provider
• gives managed requirements and other managed sources precedence over ordinary config and plugins
• emits startup warnings for ignored providers
• fails closed when a managed requirements provider cannot run

manual validation

• just test -p codex-hooks discovery
• 14 tests passed

stack, merge in order

1. hooks: define UserInstructions execution contract #30138 hooks: define UserInstructions execution contract
2. config: add singleton UserInstructions declarations #30139 config: add singleton UserInstructions declarations
3. hooks: resolve UserInstructions provider precedence #30140 hooks: resolve UserInstructions provider precedence
4. core: load hook-backed user instructions #30141 core: load hook-backed user instructions