feat(tui): add PageUp/PageDown scrolling to all panes

[major]

Summary

Add PageUp/PageDown key support to the policy, logs, and draft/rules views in openshell term. All three scrollable panes now scroll by one viewport height per keypress, matching standard terminal conventions.

Also fixes a pre-existing scroll clamping bug where pressing G (go to bottom) or scrolling past the end would overshoot into a blank screen.

Related Issue

N/A (user-reported usability gap, no tracking issue)

Changes

• Add policy_viewport_height field to App, set during draw by sandbox_policy::draw() (matches how logs/draft already store viewport height)
• Handle KeyCode::PageDown / KeyCode::PageUp in handle_policy_key(), handle_logs_key(), and handle_draft_key()
• Fix scroll_policy() max clamp to use viewport-aware bound (total - viewport) instead of total - 1
• Fix G keybinding in policy view to use the same viewport-aware clamping
• sandbox_policy::draw() takes &mut App (consistent with sandbox_logs::draw() and sandbox_draft::draw())
• Logs PageDown/PageUp disables autoscroll (consistent with j/k behavior)
• Draft PageUp guards on total > 0 (consistent with PageDown)
• Extract clamped_scroll() helper with 10 unit tests covering empty content, short/equal/long content, pre-first-draw viewport, and boundary clamping

Testing

• Built with cargo build --release -p openshell-cli
• Manual testing: verified PageUp/PageDown in policy view with 90+ rules
• Verified no blank-screen overshoot on PageDown at end of content
• LSP diagnostics clean on all changed files
• mise run pre-commit passes (rust:format:check, rust:lint, helm:lint, markdown:lint, license:check all green)
• cargo test -p openshell-tui passes (22 tests, including 10 new scroll tests)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[github-actions]

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[major]

I have read the DCO document and I hereby sign the DCO.

[major]

recheck

[johntmyers]

/ok to test ef00759