feat(lint): expand Vale audit prefilter into a broader plugin-content harness

Deferred item from PR #85 review. Per ADR-0013: cherry-picks two low-noise
rules from trialing write-good/alex against the real corpus (VagueQualifier,
SentenceOpenerThereIs) into styles/Kyberforge rather than adopting either
package wholesale (both are tuned for blog prose and were noisy on this
repo's terse, imperative instruction files - see the ADR's rejected-rule
list). Adds a new skill-size-check pre-commit hook enforcing agentskills.io's
500-line/5,000-token SKILL.md ceiling, currently unenforced. Fixes the 28
resulting violations across 20 existing SKILL.md/agent files so the
enforcing pre-commit hook lands clean.

governance.md/CONTROLS.md were evaluated and excluded as rule sources -
they're org/CI-infrastructure controls, not prose patterns Vale can express.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QUDczvw1H3eEeMD29Q9Lbi

plugins/kyberforge/skills/agent-author/SKILL.md

@@ -123,7 +123,7 @@ source_keys:
 
 ### Step 3 — Fill in the Copilot agent file
 
-There are **two distinct Copilot agent formats** with different paths and field sets. Choose one based on the deployment target:
+**Two distinct Copilot agent formats** exist, with different paths and field sets. Choose one based on the deployment target:
 
 **CLI format** (default — what the scaffold creates):
 - Path: `.github/agents/<name>.agent.md` (project) or `<plugin>/agents/<name>.agent.md` (plugin)