feat: migrate CLAUDE.md content to OpenSpec specs and initialize workflow

Move project description and tech stack research from CLAUDE.md into
openspec/specs/project/ and openspec/specs/stack/. Slim CLAUDE.md to a
pointer file. Populate config.yaml with project context. Add OpenSpec
CLI skills and commands.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

openspec/specs/stack/spec.md

@@ -0,0 +1,54 @@
+## Purpose
+
+Document the technology stack decisions for Chat Agent WebApp.
+## Requirements
+### Requirement: Core technology stack
+
+The stack spec SHALL document the recommended core technologies with version, purpose, and rationale for each.
+
+#### Scenario: Core stack documented
+
+- **WHEN** a developer needs to add or update a dependency
+- **THEN** the stack spec provides the authoritative record of: .NET 9 SDK, Blazor WebAssembly Standalone, ASP.NET Core Web API, C# 13, OpenAI SDK 2.9.1, Markdig 1.1.1, MudBlazor 9.2.0, and System.Text.Json
+
+### Requirement: Supporting libraries and tools
+
+The stack spec SHALL document supporting libraries, development tools, and installation notes.
+
+#### Scenario: Supporting libraries referenced
+
+- **WHEN** a developer evaluates adding a new dependency
+- **THEN** the stack spec lists supporting libraries with guidance on when to use them (e.g., Microsoft.Extensions.AI — skip for v1)
+
+### Requirement: Alternatives and exclusions
+
+The stack spec SHALL document considered alternatives and explicitly excluded technologies with rationale.
+
+#### Scenario: Alternative considered
+
+- **WHEN** a developer proposes an alternative package or approach
+- **THEN** the stack spec provides a record of alternatives already evaluated and why the current choice was made
+
+#### Scenario: Excluded technology referenced
+
+- **WHEN** a developer considers using a technology on the exclusion list
+- **THEN** the stack spec explains why it was excluded and what to use instead
+
+### Requirement: Stack patterns
+
+The stack spec SHALL document implementation patterns that govern how stack technologies are used together (streaming, storage, markdown rendering).
+
+#### Scenario: Pattern referenced during implementation
+
+- **WHEN** a developer implements streaming, storage, or markdown rendering
+- **THEN** the stack spec provides the canonical pattern to follow
+
+### Requirement: Version compatibility matrix
+
+The stack spec SHALL maintain a compatibility matrix and list of authoritative sources for version decisions.
+
+#### Scenario: Compatibility check
+
+- **WHEN** a package version is being upgraded
+- **THEN** the stack spec provides the compatibility matrix to verify cross-package compatibility
+