fix(tui): auto-scroll via onContentHeightChange callback

[avoidwork]

Description

Replace the messages-state-dependent scroll effect with ScrollView's onContentHeightChange callback. The previous approach tracked message count and streaming content hash, but never fired during streaming because messagesRef.current mutations don't change the messages array reference.

onContentHeightChange fires when ink-scroll-view's internal content height actually changes — driven by MeasurableItem useLayoutEffect measurements — so it catches both streaming text growth and new messages without any state churn.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

Describe the tests you wrote/run and their coverage. Use "N/A" if not applicable.

Coverage

• Line coverage maintained

Checklist

• npm run lint passes
• Tests pass with maintained line coverage
• No forbidden patterns used
• Conventional Commit style applied