Proposal: Extensible hooks and plugin support

[araujof]

Summary

Introduces a design specification for an extensible hook system and plugin framework for Mellea. The hook system provides extension points across the framework's execution lifecycle — from session initialization through component execution, LLM generation, validation, sampling, and cleanup — enabling policy enforcement, observability, customization, and integrations without modifying core library code.

Document Outline

• Plugin framework design: Async hook API with typed payloads, read-only context, and result objects that can modify payloads or block execution. Plugins register via convention-based naming (on_<hook_name>), programmatic registration, or YAML configuration.
• Hook points organized into categories that reflect Mellea's abstraction boundaries.
• Execution model: Blocking (default) and fire-and-forget modes, priority-based ordering, and enforcement modes.
• Concurrency semantics: Documents how Python's cooperative async model keeps hook execution deterministic when awaited, and where race conditions can arise.
• Observability integration: Describes how hooks compose with OTel-instrumented LLM client libraries.
• Error handling: Plugin error isolation, circuit breaking, configurable timeouts.
• Example plugins: Content policy, audit logging, token budget enforcement, and generative slot profiling.

Review plan

• Review spec for completeness against current Mellea internals
• Validate hook trigger points map to actual code paths
• Gather team feedback on hook surface area (too many? too few? missing categories?)

[mergify]

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