Implement descriptiveness ACT rules (5 rules)

## ACT rules requiring semantic/subjective evaluation

| ACT Rule | Name |
|---|---|
| `b49b2e` | Heading is descriptive |
| `c4a8a4` | HTML page title is descriptive |
| `cc0f0a` | Form field label is descriptive |
| `5effbb` | Link in context is descriptive |
| `aizyf1` | Link is descriptive |

## Blocker
These rules require understanding whether text is *descriptive enough* — a subjective judgment that cannot be reliably automated with DOM inspection alone. An LLM-based approach could work but would add significant complexity and a non-deterministic dependency.

Related to #350.

Review the five listed ACT rules and the discussion in related issue #350 first. The work is complete only when all five descriptiveness rules are implemented, but the issue does not define an objective evaluation approach for their subjective requirements.

Written by the indexing model from the issue text.