Add idle TUI footer navigation hints

[warp-dev-github-integration]

Description

Add idle-state footer navigation hints after the current conversation has at least one completed exchange. Key labels use the Figma #D0D1FE color through a semantic TUI builder style, while action labels use the muted text style.

Fixes CODE-1852

Linked Issue

https://linear.app/warpdotdev/issue/CODE-1852/tui-verify-footer-navigation-hints-missing-in-idle-state

• The linked issue is labeled ready-to-spec or ready-to-implement.
• TUI capture evidence is included below.

Testing

• cargo nextest run -p warp_tui -E 'test(idle_footer_navigation_hint_renders_key_and_action_text)'

• env -u WARP_API_KEY cargo nextest run -p warp_tui --no-fail-fast — 203 passed

• cargo nextest run -p warpui_core --no-fail-fast — 310 passed, 7 skipped

• cargo build --release -p warp_tui --bin warp-tui-dev

• ./script/format

• cargo clippy --workspace --exclude warp_completer --all-targets --tests -- -D warnings

• cargo clippy -p warp_completer --all-targets --tests -- -D warnings

• I manually tested the release warp-tui-dev binary under tmux.

TUI verification

Before, after one completed exchange, the left footer slot was empty:

∷ 4s • 9.1 credits
┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│                                                                                                                                      │
└──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
                                 auto (genius) /workspace/warp-code-1852 ↬ tui-verify/CODE-1852-footer-idle-navigation-hints • 9.1 credits

After, the completed-exchange idle footer contains the full navigation hint:

∷ 2s • 9.1 credits
┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│                                                                                                                                                                                                                      │
└──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
↑ to edit  Esc to stop  ← for conversations                                                          auto (genius) /workspace/warp-code-1852 ↬ tui-verify/CODE-1852-footer-idle-navigation-hints • 9.1 credits • +102 -7

A separate fresh zero-state capture contains no navigation hint. The ANSI capture reports 38;2;208;209;254m for each key span and muted 38;2;157;157;157m for each action span.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-BUG-FIX: Added missing navigation hints to the TUI footer after completed conversations.

Conversation: https://staging.warp.dev/conversation/29de3017-5e54-4f8f-bf25-61cb765b567d
Run: https://oz.staging.warp.dev/runs/019f62f3-4834-76e5-ac87-d0188519a0c5

Co-Authored-By: Oz oz-agent@warp.dev

This PR was generated with Oz.

[warp-dev-github-integration]

Addressed all three points of feedback:

1. Test in separate file: Moved render_idle_navigation_hint from terminal_session_view.rs into TuiUiBuilder as a method in tui_builder.rs (alongside all other semantic styling helpers). The test (idle_navigation_hint_renders_key_and_action_text) now lives in tui_builder_tests.rs with a render_buffer helper.

2. No hardcoded color: Replaced Color::Rgb(208, 209, 254) in key_hint_style with cell_color(ThemeFill::from(self.warp_theme.terminal_colors().bright.blue)) — adapts to the active terminal theme.

3. Rebased: Force-pushed onto the latest master (which included the LRC terminal-use PR and inline-menu fix). PR is now MERGEABLE.