fix(tui): route warning logs to status bar instead of stderr

[r3v5]

Summary

• Replace all 25 tracing::warn!/info!/debug!() calls in openshell-tui with app.status_text assignments to prevent stderr writes from corrupting ratatui's alternate-screen layout
• Add ForwardWarnings event variant to decouple port-forward warning delivery from sandbox name in CreateResult, preventing downstream gRPC lookup failures in handle_exec_command
• Remove tracing dependency from the TUI crate as a compile-time guard against reintroduction (14 other crates retain it)

Related Issue

Closes NVIDIA#2120

Changes

crates/openshell-tui/src/lib.rs

• 18 direct-access tracing::*!() calls → app.status_text assignments
• 7 spawned-task calls in start_port_forwards() → Vec<String> accumulation returned to caller
• 2 tracing::debug!() calls in refresh_draft_chunks() silently dropped (not user-actionable)
• start_port_forwards() return type changed from () to Vec<String>
• Forward warnings sent via Event::ForwardWarnings, not embedded in sandbox name

crates/openshell-tui/src/event.rs

• Added ForwardWarnings(Vec<String>) variant to Event enum

crates/openshell-tui/Cargo.toml

• Removed tracing = { workspace = true }

Testing

• cargo build -p openshell-tui — compiles clean
• cargo clippy -p openshell-tui — no warnings
• grep -rn 'tracing::' crates/openshell-tui/src/ — zero results
• mise run pre-commit — passes (excluding unrelated prompt.md)
• Manual: openshell term → error sandbox → clean layout, no stderr corruption
• Manual: gateway stopped mid-session → errors display in title bar, TUI intact
• Reproduced exact scenario from issue [term] Warning Log Messed Up the TUI NVIDIA/OpenShell#2120 (error-phase sandbox detail view)