Exclude markdown headings from paragraph coherence analysis

## Problem

The semantic coherence analysis treats markdown headings (`#`, `##`, `###`, etc.) as paragraphs and flags transitions between headings and content as "too disconnected."

Example from analysis:
```
Para 1->2 (sim=0.05): '# Building an Attack Surface Discovery Workflow...' -> 'Your organization owns more...'
Para 5->6 (sim=0.29): 'Attackers don't share your blind spots...' -> '## The Problem: You Can't Secure...'
```

Headings are structural labels, not content paragraphs. They will **always** have low semantic similarity to surrounding text because:
- They're short summaries, not full ideas
- They mark section boundaries, not idea continuity
- Their purpose is navigation, not discourse flow

## Proposed Solution

Before computing paragraph transitions:
1. Identify lines starting with `#` as headings (not paragraphs)
2. Exclude heading → paragraph and paragraph → heading transitions from coherence scoring
3. Only analyze transitions between actual prose paragraphs

Alternatively, treat headings as section delimiters that "reset" the coherence expectation - the first paragraph after a heading shouldn't need to connect to the last paragraph of the previous section.

## Impact

Articles with proper heading structure get penalized for having headings. This artificially lowers semantic coherence scores and triggers "weak transition" warnings for structurally correct documents.