Refactor completion gate: architecture boundaries, ADRs, and module ownership

[Hmbown]

Scope

By the final current roadmap milestone, finish the factorization thread: no duplicated cockpit/session/tool/control calculations, clear module ownership, and docs/tests that prevent drift.

Acceptance

• Add/update ADRs describing final ownership boundaries across TUI rendering, command receipts, workbench state, control plane, tool studio, continuity/cache, config/settings, and docs.
• Add lightweight tests or compile-time checks where practical so new enum variants/routes cannot be partially wired.
• Review broad refactor issue Proposal: Refactoring - Split large monolithic files into submodules #1461 and close/supersede/split it only with evidence.
• Confirm README, docs, release checklist, and contributor guide point to the new architecture.
• Final check: no major user-visible surface computes tokens/cost/tool/subagent state independently when a shared source exists.