docs: simplify tmux workspace guidance

[gracee3]

Summary

• keep new tmux panes in the existing session and window
• split or rotate the current window layout as needed
• retire legacy qwen38 layout conventions
• avoid moving panes between windows due to the observed tmux issue

Validation

• git diff --check
• just check (Rust build/tests, PTY test, and script smoke tests passed; final doctor step stopped because ffmpeg is not installed on this host)

[gracee3]

Structured handoff

• Outcome: Repository tmux guidance now keeps new panes in the existing session/window, permits splitting or rotating that window, retires legacy qwen38 conventions, and avoids cross-window pane moves.
• Exact commit: c2fef6149a7954562826898bd88e8788d951663a
• Validation: git diff --check passed. just check completed the Rust build/tests, PTY test, and script smoke tests, then stopped at the doctor step because ffmpeg is not installed on this host.
• Risks / decisions: Documentation-only change. No tmux commands or host configuration changes were made. The existing dirty feature worktree was preserved by using an isolated worktree based on origin/main.
• Immediate next action: Review and merge PR docs: simplify tmux workspace guidance #5. External portfolio coordination still needs verification by an identity with GitHub Projects read access.