fix: Show mode-specific instructions section with placeholder when empty (#5468)

[roomote]

Description

Fixes #5468

This PR resolves the bug where the "Mode-specific Custom Instructions" section disappears entirely from the system prompt when the field is empty or reset to default, instead of showing helpful placeholder text.

Changes Made

• Modified addCustomInstructions: Changed conditional logic to always show mode-specific instruction sections when a string is provided, regardless of whether it's empty
• Added placeholder text: When mode-specific instructions are empty, the section now shows "(No mode-specific instructions set)" instead of being omitted entirely
• Updated test case: Modified the test "should return empty string when no instructions provided" to expect placeholder text instead of empty string, reflecting the new correct behavior

Technical Details

Before

if (typeof modeCustomInstructions === "string" && modeCustomInstructions.trim()) {
    sections.push(`Mode-specific Instructions:\n${modeCustomInstructions.trim()}`)
}

After

if (typeof modeCustomInstructions === "string") {
    const trimmed = modeCustomInstructions.trim()
    sections.push(`Mode-specific Instructions:\n${trimmed || "(No mode-specific instructions set)"}`)
}

Testing

• All existing tests pass
• Updated test case to reflect new expected behavior
• All prompt-related tests pass (12 test files, 104 tests)
• Manual verification that the fix addresses the issue
• Linting and type checking pass

Verification of Acceptance Criteria

• Empty mode-specific instructions now show section: The "Mode-specific Instructions" section appears in the system prompt even when the field is empty
• Helpful placeholder text: Empty sections show "(No mode-specific instructions set)" instead of being omitted
• Consistent behavior: Both global and mode-specific instruction sections now behave consistently
• No breaking changes: Existing functionality with non-empty instructions works exactly as before

Impact

This change improves user experience by:

• Making instruction sections visible even when empty, providing clarity about what sections exist
• Showing helpful placeholder text that explains the current state
• Maintaining consistency between global and mode-specific instruction handling
• Ensuring users understand that the section exists and can be configured

Files Changed

• src/core/prompts/sections/custom-instructions.ts - Core logic fix
• src/core/prompts/sections/__tests__/custom-instructions.spec.ts - Updated test expectations

---

Important

Fixes bug #5468 by ensuring mode-specific instructions section is always shown with placeholder text when empty.

• Behavior:
  • addCustomInstructions in custom-instructions.ts now always shows mode-specific instruction sections, even if empty.
  • Adds placeholder text "(No mode-specific instructions set)" when instructions are empty.
• Testing:
  • Updates test "should return empty string when no instructions provided" in custom-instructions.spec.ts to expect placeholder text.
  • All prompt-related tests pass (12 test files, 104 tests).
• Misc:
  • Fixes bug Bug: Mode-specific Custom Instructions section missing from system prompt when field is empty/default (Regression from PR #3791) #5468 where mode-specific instructions section disappeared when empty.

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[delve-auditor]

✅ No security or compliance issues detected. Reviewed everything up to eee9644.

Security Overview

• 🔎 Scanned files: 2 changed file(s)

Detected Code Changes

Change Type	Relevant files
Enhancement	► custom-instructions.ts     Add placeholder text for empty instruction sections     Show mode-specific sections even when empty ► custom-instructions.spec.ts     Update tests to expect placeholder text

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