docs(workhub): record coordination session architecture

[ARE404]

Summary

• Record the accepted one-stable-WorkHub-Coordination-Session-per-Runtime-Host decision.
• Align the WorkHub domain language and documentation authority map with that decision.

Refs #3492

Authority and deferred decisions

• Session remains the only durable substrate: the Coordination Session owns the coordination conversation, ordinary Sessions own concrete execution, and projections own no durable facts.
• Model/router output is advisory; writes require the deterministic Action Gate. Delegation stores bounded links instead of transcript copies.
• Work cardinality and cross-Host coordination remain deferred. This PR adds no Slice 2 lifecycle, recovery, UI, or routing implementation.

Verification

• npm run lint
• npm run format:check
• npm run check:asf-headers
• git diff --check
• Standards and Spec adversarial reviews: no remaining findings.
• GitHub CI test: passed.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex drafted and reviewed the ADR, domain-language alignment, and documentation index; the commit includes Generated-by: Codex.

Checklist

• Tests cover the change and fail without it — not applicable to this documentation-only change.
• Lint, format, typecheck and the affected suites pass locally — lint and format pass; typecheck and test suites were not run for this documentation-only change.

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[Astro-Han]

APPROVE — e1fd5fe, docs-only, no P0-P2 per review, merging per owner.