feat: enforce absolute ban on em dashes and en dashes

[erhanurgun]

Closes #78

Summary

• Replaced "overuse" warning with an absolute ban on em dashes (—) and en dashes (–)
• Added replacement hierarchy (period > comma > colon > semicolon > parentheses > rewrite)
• Added mandatory em dash scan step to the workflow
• Added zero-dash constraint to output format

Motivation

The em dash has become one of the most reliable tells of AI-generated writing. A "use sparingly" approach was insufficient; a complete ban aligns with humanizer's core purpose.

Changes

• SKILL.md: Rewrote Rule 14, added self-check step and output constraint
• Version: 2.5.1 → 2.6.0

Test plan

• Run humanizer skill on text containing em dashes
• Verify output contains zero — and – characters
• Confirm replacement hierarchy is applied correctly

[erhanurgun]

Note on overlapping PRs

This PR overlaps with #85 and #84, which also modify Rule 14 in SKILL.md:

• Add patterns 30-31, expand rules 9 and 14 (v2.6.0) #85 expands Rule 14 with paired em dash bracketing (keeps the "overuse" framing)
• fix: add content preservation rule and strengthen em dash handling #84 adds spaced em dash and double-hyphen variants to Rule 14 (also keeps "overuse" framing)

Both of those PRs build on the existing "use sparingly" approach. This PR takes a stricter stance: a complete ban with a replacement hierarchy and a mandatory scan step before output.

I believe an absolute ban is the right call here. Issue #78 shows that the "overuse" framing still lets em dashes slip through. A hard constraint with a self-check step is more enforceable than a soft guideline.

If this PR is merged first, #85 and #84 would need to be rebased and reconciled against the new Rule 14 !!!