P5.2: Remove metadata.tmux_session_name alias (multiplexer Phase 5)

[takemo101]

Phase 5, issue P5.2 — 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.

Outcome

metadata.tmux_session_name is no longer written; readers use pane_session_name (or the structured pane_handle if introduced).

Files

• Modify: packages/adapters/src/pane-adapter.ts
• Modify: packages/tui/src/attach.ts (drop fallback)
• Modify: CHANGELOG.md (### Removed)

Acceptance

No remaining references to tmux_session_name.

[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.