refactor: migrate docs from Mintlify to Astro Starlight

- Move introduction.mdx to src/content/docs/index.mdx
- Move specification/*.mdx to src/content/docs/specification/
- Move design/rationale.mdx to src/content/docs/design/
- Move logos to src/assets/, favicon to public/
- Remove docs.json Mintlify config
- Fix MDX parse errors for angle brackets in content
- Add dist/ and .astro/ to .gitignore

Author: zircote

.gitignore

@@ -7,3 +7,5 @@ node_modules/
 Gemfile.lock
 .DS_Store
 *.ccpkg
+dist/
+.astro/

docs.json

@@ -21,7 +21,7 @@ Skills conform to the [Agent Skills specification](https://agentskills.io/specif
 Skills support three-tier progressive disclosure to minimize context token usage:
 
 1. **Metadata** (~100 tokens): The `name` and `description` from frontmatter, loaded at session startup for all installed skills.
-2. **Instructions** (<5000 tokens recommended): The full `SKILL.md` body, loaded when the skill is activated.
+2. **Instructions** (&lt;5000 tokens recommended): The full `SKILL.md` body, loaded when the skill is activated.
 3. **Resources** (as needed): Files in `scripts/`, `references/`, and `assets/`, loaded only when required during execution.
 
 **Example SKILL.md:**
@@ -39,10 +39,7 @@ Validate OpenAPI specification files for correctness and compliance.
 ## Usage
 
 1. Read the target OpenAPI spec file (YAML or JSON).
-2. Run the validation script:
-   ```
-   scripts/validate.py <spec-file>
-   ```
+2. Run the validation script: `scripts/validate.py <spec-file>`
 3. Report findings grouped by severity (error, warning, info).
 
 ## Edge Cases