docs: add OpenCode AI-assisted development guide (#2384)

Adds OpenCode support for AI-assisted development, including custom
commands and skills to help contributors maintain consistency and
streamline common workflows.

#### Changes
- Added "AI-Assisted Development with OpenCode" section to
CONTRIBUTING.md with:
  - Installation instructions and provider configuration
- Documentation for 8 custom commands (/implement, /continue,
/interview, /document, /commit, /create-plan, /create-scratch,
/create-justification)
  - Typical workflow guide
- Clear policy that AI-generated code must be reviewed before submission
- Added .agents/ directory for plans, scratches, and justifications
- Added .opencode/ commands and skills for the agent
- Added helper scripts for creating agent files

.opencode/skill/create-scratch/SKILL.md

@@ -0,0 +1,56 @@
+---
+name: create-scratch
+description: Create a new scratch file in .agents/scratches/ with a unique three-word ID, frontmatter, and formatted title
+license: MIT
+compatibility: opencode
+metadata:
+  audience: agents
+  workflow: exploration
+---
+
+## What I do
+
+I help you create new scratch files in the `.agents/scratches/` directory. Each scratch file gets:
+
+- A unique three-word identifier (e.g., `calm-teal-cloud`)
+- Frontmatter with the current date and formatted title
+- Content you provide
+
+## How to use
+
+Run the script with a slug and content:
+
+```bash
+npx tsx scripts/create-scratch.ts "note-name" "Scratch content here"
+```
+
+Or use heredoc for multi-line content:
+
+```bash
+npx tsx scripts/create-scratch.ts "note-name" << HEREDOC
+Multi-line
+scratch content
+goes here
+HEREDOC
+```
+
+## File format
+
+Files are created as: `{three-word-id}-{slug}.md`
+
+Example: `calm-teal-cloud-note-name.md`
+
+The file includes frontmatter:
+
+```markdown
+---
+date: 2026-01-13
+title: Note Name
+---
+
+Your content here
+```
+
+## When to use me
+
+Use this skill when you need to create a temporary note, exploration document, or scratch pad for ideas. The unique ID ensures no filename conflicts, and the frontmatter provides metadata for organization.