Add HK IPO audit skill metadata

Request:
- Add missing agents/openai.yaml metadata for the hk-ipo-audit skill.

Changes:
- Added UI-facing display name, short description, and default prompt for hk-ipo-audit.
- Enabled implicit invocation to match the analyst and archivist skills.

Verification:
- Ran git diff --check and git diff --cached --check.
- Confirmed the new openai.yaml content matches the existing project skill metadata style.

Next useful context:
- The prior local commit that moved skills from .codex to .agents was already ahead of origin before this change.

.agents/skills/hk-ipo-audit/agents/openai.yaml

@@ -0,0 +1,7 @@
+interface:
+  display_name: "hk-ipo-audit"
+  short_description: "Audit HK IPO data and analysis quality"
+  default_prompt: "Use $hk-ipo-audit to audit HK IPO archive completeness and analysis logic."
+
+policy:
+  allow_implicit_invocation: true