feat(citation-validator): warn when markdown links point to folders

[WesleyMFrederick]

Problem

Citation-manager validate currently passes folder links as valid citations. While these are syntactically correct markdown links, they cause issues in practice:

• Obsidian: Cannot preview folder links (shows broken preview)
• User expectation: Links should point to readable content
• False positives: Invalid links pass validation

Reproduction Steps

1. Validate a file containing a folder link:

- [Research Examples](research/) - Sample interviews and brick extractions

2. Observe validation passes with no warnings:

Citation Validation Report
==========================
VALID CITATIONS (2)
├─ Line 99: [Architecture Principles](../ARCHITECTURE-PRINCIPLES.md)
└─ Line 100: [Full Architecture](ARCHITECTURE-Evidence-Tracker.md)

ALL CITATIONS VALID

Note: The folder link research/ at line 101 is not even detected/reported.

Proposed Solution

Add validation check for folder links:

if (fs.statSync(resolvedPath).isDirectory()) {
  warnings.push({
    line,
    type: 'folder-link',
    message: 'Link points to folder, not file. Obsidian cannot preview folders.',
    suggestion: 'Link to a specific file or create an index.md'
  });
}

Impact

• Severity: Medium - causes UX issues but has workaround
• Frequency: Common in documentation structure
• Detection: Silent failure (passes validation)

Related

• Component: CitationValidator
• Tool: citation-manager
• Context: PostToolUse hook validation

---

Acceptance Criteria

• Validator detects links pointing to directories (e.g., research/, ./docs/)
• Folder links emit a warning (not error) with actionable suggestion
• Warning message includes suggestion to link to specific file or create index.md
• Non-folder links continue to validate without false warnings
• Trailing slash and no-trailing-slash folder paths both detected

Definition of Done

• Failing test written: folder link passes validation without warning (RED phase)
• Fix implemented: directory detection added to validation pipeline (GREEN phase)
• All existing citation-manager tests pass
• Build succeeds: npm run build -w tools/citation-manager && npm link -w tools/citation-manager
• Manual verification: folder link in test file emits warning
• Committed with conventional commit message referencing this issue