feat(tui): replace tool buttons with key hints

[edwin-zvs]

Summary

• replace long-running zarvis tool-block [bg]/[kill] labels with keyboard hints: Ctrl-b background, Esc kill
• keep Esc behavior via the existing zarvis interrupt path and wire Ctrl-b to background the active tool supervisor
• stop exposing button hit zones for newly rendered text-hint tool blocks

Tests

• cargo test -p agentd-cli tool_block_shows -- --nocapture
• cargo test -p agentd-adapter-zarvis pty_background_detection_only_matches_ctrl_b -- --nocapture
• cargo test -p agentd-cli
• cargo test -p agentd-adapter-zarvis
• cargo build
• git diff --check

Relevant binaries: agent renders the tool block hints; agentd-adapter-zarvis handles Ctrl-b while a tool is running.