fix(tui): support cursor navigation in text areas

[notgitika]

Summary

• make FormTextArea cursor-aware instead of append-only
• support Left/Right movement and insertion, newline, and backspace at the cursor
• keep the active cursor line visible in multiline previews
• add a Harness wizard regression test covering both arrow directions

Reproduction

In the refactor RC, run agentcore add harness and enter a system prompt. Pressing Left or Right does not move the cursor, so earlier text cannot be corrected in place.

Validation

• bun test src/handlers/project/add/harness/harness.screen.test.tsx
• bun test src/components/wizard/wizard.test.tsx src/handlers/project/add/credentials/oauth/oauth.screen.test.tsx
• bun run typecheck
• oxlint and Prettier checks on changed files

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[agentcore-devx-automation]

AgentCore Harness Review

Verdict: Looks good

The cursor-aware navigation for FormTextArea is a clean refactor of what was previously an append-only editor. A few things I checked and found fine:

• rawCursor is clamped via Math.min(rawCursor, value.length) on every render, so external value changes (including reset to "") don't leave the cursor out of range.
• cursorLine / cursorColumn math is correct at line boundaries — placing the cursor at the end of a middle line renders as " " via line[cursorColumn] ?? " ", and empty cursor lines still take one row via the inverse-space cursor.
• Swallowing up/down is intentional (they'd otherwise leak to any parent handler as a would-be advance/back); since ink runs all active useInput handlers, this doesn't break the wizard's escape/ctrl+d handling in TextAreaField.
• No other component in src/ uses left/right in a way that overlaps with a focused FormTextArea.
• New behavior is covered by harness.screen.test.tsx using real key presses through the test harness — no mocking concerns.

Nothing to change before merging.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.87755% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.39%. Comparing base (04b45c3) to head (ef7295c).

Files with missing lines	Patch %	Lines
src/components/FormTextArea.tsx	93.87%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           refactor    #2488      +/-   ##
============================================
- Coverage     97.39%   97.39%   -0.01%     
============================================
  Files           642      642              
  Lines         46807    46844      +37     
============================================
+ Hits          45590    45625      +35     
- Misses         1217     1219       +2     

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