feat(plugins): add trigger-oriented skill activation system

[moukrea]

Summary

• Rewrite all 9 skill descriptions (opaq + git-pilot) from passive capability statements to trigger-oriented language (USE THIS SKILL WHEN...) per Anthropic's skill-creator guidance, solving Claude's tendency to under-trigger skills
• Create new git-pilot-workflow model-invocable skill (160 lines, 7 sections) as the behavioral contract entry point that activates on any coding task — even when users don't mention git
• Create opaq/CLAUDE.md with 5 persistent behavioral rules for credential management, plus session-start.sh (availability check + directive message) and post-bash-auth-check.sh (401/403 detection + credential suggestion)
• Strengthen all 5 git-pilot hook scripts from informational to directive language (You MUST, STOP and use AskUserQuestion NOW, Before continuing)
• Add user-invocable: true to all 7 git-pilot sub-skills and cross-reference header to CLAUDE.md establishing skill precedence
• Update opaq/hooks/hooks.json to use script-based SessionStart and add PostToolUse Bash hook for auth-failure detection

Test plan

• Verify shellcheck plugins/opaq/scripts/*.sh passes with no errors
• Verify shellcheck plugins/git-pilot/scripts/*.sh passes with no errors (warnings acceptable for existing code)
• Verify .github/scripts/validate-plugins.sh passes
• Verify all 9 skill descriptions start with USE THIS SKILL WHEN (no capability verbs)
• Verify all git-pilot sub-skills have user-invocable: true and no disable-model-invocation
• Verify git-pilot-workflow skill body is 150-250 lines with 7 sections
• Verify opaq/CLAUDE.md has 5 rules and skill reference
• Verify opaq/hooks/hooks.json references session-start.sh and includes PostToolUse Bash hook