Document async scheduling architecture, capacity, and operations

Priority Level

Medium

Epic: #645
Depends on: #652, #654, #657, #648, #649
Related: #641, #644, #635, #647, #650, #651
Target branch: epic/645-async-scheduling while the epic is active.

Source of Truth

Specification details live in PR #658 under plans/645/, especially README.md, architecture.md, contracts.md, capacity-model.md, observability.md, benchmark-plan.md, migration-and-cleanup.md, and issue-map.md. This issue tracks the final docs slice and gates only.

Documentation Scope

Document the complete async scheduling architecture after the metadata, task-admission, request-admission, capacity, observability, and benchmark issues land.

This issue owns:

• maintainer/developer docs for the two-stage task/request scheduling model;
• operator docs for capacity layers, runtime snapshots, and telemetry interpretation;
• links from plugin-facing SchedulingMetadata docs (#652) without exposing scheduler internals as plugin API;
• migration notes for removed names and their replacements;
• docs references to benchmark artifacts and how to read them.

This issue does not own the plugin-facing SchedulingMetadata guide itself; #652 owns that narrower documentation.

Quality Gates

• Docs match the UML-level architecture and terminology in plans/645.
• Task admission, request admission, capacity, observability, and benchmarks are explained as separate but correlated layers.
• Docs do not present removed Throttle*, legacy hint resolver, or old LLM-wait names as current durable APIs.
• Public/plugin docs do not expose scheduler queue/admission internals as extension points.
• Cross-links cover the relevant epic issues and plan sections.

Validation

Run the repository docs build/preview workflow for the touched surface, make check if required by the docs workflow, link checks if available, and stale-term searches over docs/current package docs for the removed names listed in plans/645/migration-and-cleanup.md.

Start with PR #658 and the specified files under plans/645/, especially README.md, architecture.md, contracts.md, capacity-model.md, observability.md, benchmark-plan.md, migration-and-cleanup.md, and issue-map.md. After the dependent epic issues land, update the maintainer, operator, and migration documentation, then run the repository docs build/preview workflow, make check if required, link checks, and stale-term searches; done means the quality gates are satisfied.