Assess and simplify BTW module responsibilities

[ladislas]

Summary

Assess and simplify BTW module responsibilities. Priority: P3.

Motivation

The roughly 1,500-line entrypoint combines overlay behavior, persistence, imports, and model/session lifecycle work. Size alone does not justify a refactor; assess whether responsibility boundaries are causing concrete change or test friction.

Reviewed source: extensions/btw/index.ts. Revalidate against current main before implementation.

Acceptance Criteria

• Map the current responsibilities and identify concrete coupling or testing problems before proposing extraction.
• Choose and implement a small coherent extraction only when it improves ownership or testing; keep the entrypoint as appropriate orchestration.
• Preserve sidechat persistence per main session, launch-time import semantics, summaries, cancellation, restoration/UI behavior, and model/auth routing.
• Use lifecycle coverage across the changed boundary and record the architectural rationale and validation, or a reasoned no-change disposition.

Scope and verification

Preserve the extension-per-directory structure and avoid a general extension framework. Current-model display (#124) and task-aware model routing (#390) remain separate features.

Run affected regression coverage and required repository checks for any implementation; update relevant documentation/changelog under repository conventions. Record measurements and decisions durably in this issue or its linked PR.

Type

HITL — requires a policy or design review informed by the investigation before committing to the implementation approach. Investigation can proceed autonomously; publication follows existing authorization rules.

Parent

#466

Blocked by

None — investigation can start immediately; implement after the policy/design review described above.