Add placeholder text to planning tab message input

[jbrinkman]

Summary

This PR adds helpful placeholder text to the planning tab's message input while solving the virtual cursor artifact issue that previously caused a 'T' character to appear.

What Changed

• Added Placeholder field to Styles struct in internal/tui/styles.go
• Set placeholder text to 'ask a question or describe a task' in NewPlanningTabWithSession()
• Disabled virtual cursor with ti.SetVirtualCursor(false) to prevent character artifacts
• Applied placeholder styling using existing TextMuted color for theme consistency
• Maintained compatibility with existing custom prompt rendering system

Key Files Modified

• internal/tui/styles.go - Added placeholder styling to theme system
• internal/tui/planning_tab.go - Configured textinput with placeholder and disabled virtual cursor

Problem Solved

Previously, the planning tab showed a blank input with no guidance for users. The original placeholder text was removed because the bubbles/v2 virtual cursor rendered the first placeholder character as a cursor glyph, causing a visual 'T' artifact. This implementation restores user guidance while preventing the artifact.

Validation

All acceptance criteria verified and QA checks passed for formatting, sync, linting, and build.

Closes #231

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 5d316d10-bffd-4dbe-9e7a-61b2171d3e63

📥 Commits

Reviewing files that changed from the base of the PR and between 93f61c9 and 6fd6eed.

📒 Files selected for processing (3)

• .kiro-krew/specs/issue-231-add-placeholder-text.md
• internal/tui/planning_tab.go
• internal/tui/styles.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch spec/issue-231-61844

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