feat: add visor-agent-dev skill for AI coding assistants

Cross-tool skill (Claude Code, Codex, OpenCode) that guides developers
through the full visor assistant development workflow: writing YAML tests,
config validation, mock/real provider testing, interactive --message testing,
task trace debugging, and response evaluation.

Includes safety guardrails to prevent destructive actions, unauthorized
real provider usage, and secret leakage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: buger

.agents/skills/visor-agent-dev

@@ -0,0 +1 @@
+../../.claude/skills/visor-agent-dev