P5.1: Remove deprecated attach_hint string field (multiplexer Phase 5)

[takemo101]

Phase 5, issue P5.1 — part of the multiplexer backend abstraction implementation plan.

Design: docs/designs/cuekit-multiplexer-backend-design.md

Prerequisites: Phase 2 has been in a tagged release for at least one minor version. Do not start until that condition is met.

Outcome

attach_hint is removed from TaskStatusView. All consumers use attach_command.

Files

• Modify: packages/core/src/task-status-view.ts
• Modify: packages/tui/src/attach.ts (drop fallback)
• Modify: packages/cli/src/doctor.ts (if it referenced attach_hint)
• Modify: CHANGELOG.md (### Removed)

Acceptance

No remaining references to attach_hint in the codebase.

[takemo101]

Deferring this Phase 5 cleanup until after v0.0.5 has had a release window. Keep the compatibility surface for now so the zellij/TUI stabilization release remains easy to validate and downstream users have time to move off the deprecated shape. Revisit in v0.0.6+; do not implement in the immediate v0.0.5 follow-up unless a concrete breakage requires it.