fix: hard-fail em-dash overuse

[abhinavgautam01]

Summary

Fixes #1

This makes em-dash overuse a hard tells failure instead of an advisory warning.

Changes

• Treat em-dash overuse as a hard tell in validate_voice.py
• Make the tells CLI exit non-zero when the em-dash cap is exceeded
• Keep the existing rate-and-count guard so a single em dash in a short passage does not fail
• Update validator selftests to assert em-dash overuse is hard
• Update docs to match the new hard-fail behavior

Verification

• python3 skills/writer-style/tools/test_tools.py
• focused CLI check: over-cap em-dashes return [HARD], GATE: FAIL, exit 1
• focused CLI check: a single em dash still returns GATE: PASS, exit 0
• git diff --check
• npm_config_cache=/tmp/writer-style-npm-cache npm pack --dry-run