feat: centralized FocusMode state machine for TUI input dispatch

[yishuiliunian]

Summary

• Introduce FocusMode enum (Input / AgentPanel) to replace implicit conditional checks for agent panel navigation
• Agent panel now scrolls with ↑ N more / ↓ N more indicators when >5 agents
• 46 new test cases covering mode transitions, key dispatch, and scroll logic

Changes

New type: FocusMode in app/types.rs — orthogonal to focused_agent (mode = which region, focus = which agent)

Input dispatch refactor (input/mod.rs):

• handle_normal_key split into handle_agent_panel_key / handle_input_mode_key
• AgentPanel: Up/Down navigate agents, Enter drills in, Tab/Esc exit, Char/Backspace auto-switch to Input
• Ctrl+P/N are mode-aware (agent nav in AgentPanel, history in Input)

Dispatch logic (key_dispatch_ops.rs):

• enter_agent_panel: validates live agents, recovers stale focused_agent
• cycle_agent_focus: auto-exits AgentPanel when no agents remain
• adjust_agent_scroll: keeps focused agent in visible 5-row window

Rendering (views/agent_panel.rs):

• Sliding window render with ↑ N more / ↓ N more scroll indicators
• panel_height uses clamped offset for layout consistency

Tests (4 new files):

• focus_mode_test.rs — mode transition coverage
• focus_panel_keys_test.rs — key behavior per mode
• enter_panel_test.rs — enter_agent_panel dispatch logic
• cycle_focus_test.rs — cycling, wrap-around, scroll boundaries

Test plan

• bazel build //... passes
• bazel build //... --config=clippy zero warnings
• bazel test //... — 46/46 pass
• CI passes