Add doublecheck plugin: three-layer verification pipeline for AI output (#978)

* Add doublecheck plugin: three-layer verification pipeline for AI output

Adds a new plugin that helps users verify AI-generated content before
acting on it. Designed for sensitive contexts (legal, medical, financial,
compliance) where hallucinations carry real consequences.

Three verification layers:
- Self-Audit: extracts verifiable claims, checks internal consistency
- Source Verification: web searches per claim, produces URLs for human review
- Adversarial Review: assumes errors exist, checks hallucination patterns

Supports persistent mode (auto-verifies every factual response inline)
and one-shot mode (full report on specific text). Confidence ratings:
VERIFIED, PLAUSIBLE, UNVERIFIED, DISPUTED, FABRICATION RISK.

Includes:
- Skill (skills/doublecheck/) with bundled report template
- Agent (agents/doublecheck.agent.md) for interactive verification
- Plugin package (plugins/doublecheck/) bundling both

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address review: fix tools YAML format, remove materialized artifacts

- Fix tools frontmatter in agents/doublecheck.agent.md to use standard
  YAML list format instead of flow sequence with trailing comma
- Remove plugins/doublecheck/agents/ and plugins/doublecheck/skills/
  from tracking; these paths are in .gitignore as CI-materialized
  artifacts that should not be committed

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

plugins/doublecheck/.github/plugin/plugin.json

@@ -0,0 +1,24 @@
+{
+  "name": "doublecheck",
+  "description": "Three-layer verification pipeline for AI output. Extracts claims, finds sources, and flags hallucination risks so humans can verify before acting.",
+  "version": "1.0.0",
+  "author": {
+    "name": "Awesome Copilot Community"
+  },
+  "repository": "https://github.com/github/awesome-copilot",
+  "license": "MIT",
+  "keywords": [
+    "verification",
+    "hallucination",
+    "fact-check",
+    "source-citation",
+    "trust",
+    "safety"
+  ],
+  "agents": [
+    "./agents/doublecheck.md"
+  ],
+  "skills": [
+    "./skills/doublecheck/"
+  ]
+}