refactor: migrate TUI from Ink to OpenTUI with Bun runtime

[avoidwork]

Description

Migrate the TUI (Terminal User Interface) from Ink to OpenTUI. Replaces ink, ink-scroll-view, marked, and marked-terminal with @opentui/core and @opentui/react (plus react peer dependency). The interactive TUI now requires Bun at runtime.

Type of Change

• Breaking change (fix or feature which would cause existing functionality to not work as expected)

Breaking Changes

• node index.js --mode interactive no longer works; use bun index.js --mode interactive
• Removed dependencies: ink, ink-scroll-view, marked, marked-terminal
• Added dependencies: @opentui/core, @opentui/react, react
• JSX element names changed from PascalCase (<Box>, <Text>) to lowercase (<box>, <text>)
• Color props changed from named strings ("green") to hex values ("#00FF00")
• Key bindings changed from boolean props (key.upArrow) to canonical names (key.name === "up")
• Hand-rolled scroll utilities removed (replaced by native <scrollbox stickyScroll>)
• Blinking cursor animation removed (static cursor display only)
• marked + marked-terminal replaced by native <markdown> component

Testing

• 832 unit tests pass via node --test (Node.js for tooling/lint)
• TUI component tests run via bun test (Bun for JSX support)
• oxlint and oxfmt pass with no warnings

Coverage

• 100% line coverage maintained (TUI files excluded from Node coverage run)