Remove TUI pinned session view

[edwin-zvs]

Summary

• remove the TUI session pin marker, pin strip, pin controls, and pin-only hydration/redraw paths
• use split panes as the persistent multi-session view
• preserve shared session pinning for the web UI, Playbook cards, and dynamic widgets
• supersede the pin-tile parser spec and document the new TUI behavior

Verification

• cargo test -p construct-cli --bin construct -- --skip color::tests::truecolor_backend_still_emits_rgb (1304 passed)
• cargo build
• The full CLI suite has one unrelated baseline failure: color::tests::truecolor_backend_still_emits_rgb

Relevant binary: construct only.

Debug binary: /Users/moon/agentd/.claude/worktrees/remove-tui-pin/target/debug/construct