Add security failure skill

[amrit110]

This pull request adds a new skill for handling pip-audit security failures and updates the agent's workflow and user-facing documentation to provide clearer, more robust handling of dependency vulnerabilities. The main focus is on ensuring that the bot only attempts to fix vulnerabilities when a patched version is available and gracefully exits with an explanatory comment when no fix exists upstream.

Security vulnerability handling improvements:

• Added a new skill /fix-security-failures with detailed instructions for triaging pip-audit findings, updating dependencies when possible, and posting a PR comment (without making changes) if no patched version is available. (.claude/skills/fix-security-failures/SKILL.md)
• Updated the agent's prompt and workflow to reference the new /fix-security-failures skill for pip-audit/CVE failures, ensuring the correct process is followed. (src/aieng_bot/agent_fixer/prompts.py) [1] [2]
• Improved the summary and step-by-step explanation of security fixes in the dashboard UI, clarifying when the bot can auto-fix and when human intervention is required. (dashboard/components/failure-analysis.tsx) [1] [2]

Testing and context updates:

• Updated tests to check that the new /fix-security-failures skill is included in the agent's prompt, ensuring coverage for the new workflow. (tests/agent_fixer/test_fixer.py)