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>

docs/README.plugins.md

@@ -38,6 +38,7 @@ See [CONTRIBUTING.md](../CONTRIBUTING.md#adding-plugins) for guidelines on how t
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+| [doublecheck](../plugins/doublecheck/README.md) | Three-layer verification pipeline for AI output. Extracts claims, finds sources, and flags hallucination risks so humans can verify before acting. | 2 items | verification, hallucination, fact-check, source-citation, trust, safety |
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