feat(skills): add arc skill for composable LLM agent orchestration

Add the arc skill — a declarative orchestration framework that chains
existing Copilot skills into reproducible, reviewable development
workflows via .arc.yml template files.

Includes:
- SKILL.md with full execution lifecycle documentation
- 3 example arcs: feature-flow, bugfix-flow, quick-fix
- 3 reference docs: file-format, expression-syntax, acceptance-criteria

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

Author: pconnelly-nuance

docs/README.skills.md

@@ -35,6 +35,7 @@ See [CONTRIBUTING.md](../CONTRIBUTING.md#adding-skills) for guidelines on how to
 | [ai-prompt-engineering-safety-review](../skills/ai-prompt-engineering-safety-review/SKILL.md) | Comprehensive AI prompt engineering safety review and improvement prompt. Analyzes prompts for safety, bias, security vulnerabilities, and effectiveness while providing detailed improvement recommendations with extensive frameworks, testing methodologies, and educational content. | None |
 | [appinsights-instrumentation](../skills/appinsights-instrumentation/SKILL.md) | Instrument a webapp to send useful telemetry data to Azure App Insights | `LICENSE.txt`<br />`examples`<br />`references/ASPNETCORE.md`<br />`references/AUTO.md`<br />`references/NODEJS.md`<br />`references/PYTHON.md`<br />`scripts/appinsights.ps1` |
 | [apple-appstore-reviewer](../skills/apple-appstore-reviewer/SKILL.md) | Serves as a reviewer of the codebase with instructions on looking for Apple App Store optimizations or rejection reasons. | None |
+| [arc](../skills/arc/SKILL.md) | Composable LLM agent orchestration via .arc.yml files. Reads declarative arc templates, generates reviewable execution plans (traces), and runs steps sequentially using existing skills as primitives. Use when orchestrating multi-skill development tasks, automating feature arcs, or creating reproducible development processes. Trigger: 'run arc', 'execute arc', 'arc', 'orchestrate skills', 'run the feature flow'. | `examples`<br />`references/acceptance-criteria.md`<br />`references/expression-syntax.md`<br />`references/file-format.md` |
 | [arch-linux-triage](../skills/arch-linux-triage/SKILL.md) | Triage and resolve Arch Linux issues with pacman, systemd, and rolling-release best practices. | None |
 | [architecture-blueprint-generator](../skills/architecture-blueprint-generator/SKILL.md) | Comprehensive project architecture blueprint generator that analyzes codebases to create detailed architectural documentation. Automatically detects technology stacks and architectural patterns, generates visual diagrams, documents implementation patterns, and provides extensible blueprints for maintaining architectural consistency and guiding new development. | None |
 | [arize-ai-provider-integration](../skills/arize-ai-provider-integration/SKILL.md) | INVOKE THIS SKILL when creating, reading, updating, or deleting Arize AI integrations. Covers listing integrations, creating integrations for any supported LLM provider (OpenAI, Anthropic, Azure OpenAI, AWS Bedrock, Vertex AI, Gemini, NVIDIA NIM, custom), updating credentials or metadata, and deleting integrations using the ax CLI. | `references/ax-profiles.md`<br />`references/ax-setup.md` |