feat: add TUI slash-command palette and screen routing

[huntharo]

Summary

• add a global slash-command palette anchored in the lower-left corner of the TUI
• route existing hotkey actions through a shared command registry and reserve / for the palette
• introduce route-like screen metadata so the current clusters explorer and a future user explorer share the same navigation model

Details

• add commands.ts with command definitions, filtering, alias matching, and exact-command execution helpers
• refactor the TUI root to own active screen state, command palette state, and shared command dispatch
• add a /users screen route with a placeholder explorer so future user-centric work can plug into the same chrome cleanly
• update help/footer copy to lead with slash commands while keeping legacy hotkeys as aliases
• expand TUI tests to cover command ranking, screen metadata, footer/help text, and the user explorer placeholder

Verification

• pnpm --filter ghcrawl test -- src/tui/app.test.ts src/tui/state.test.ts src/tui/layout.test.ts src/tui/commands.test.ts
• pnpm build

Notes

• pnpm --filter ghcrawl typecheck still fails because the existing CLI no-emit typecheck path does not resolve workspace @ghcrawl/api-core imports, even after a successful workspace build. This PR does not change that setup.

[github-actions]

Cluster Performance

• Status: PASS
• Fixture median: 480.6 ms (12 samples, 3 cluster rebuilds/sample)
• Fixture baseline: 535.1 ms
• Fixture delta: -54.5 ms (-10.2%)
• Projected openclaw/openclaw duration: 8m 58.9s
• Projected openclaw/openclaw baseline: 10m 0.0s
• Projected delta: -61134.3 ms (-10.2%)
• Regression threshold: +50.0%
• Fixture shape: 512 threads x 3 source kinds
• Sample durations: 696.6 ms, 1.21 s, 487.3 ms, 485.9 ms, 473.3 ms, 472.9 ms, 485.2 ms, 479.0 ms, 476.3 ms, 477.5 ms, 482.2 ms, 475.6 ms

Run: workflow run for 00f586f