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/commands/interview.md

@@ -0,0 +1,57 @@
+---
+description: Deep-dive interview to flesh out a spec or design document
+agent: build
+argument-hint: <file-path>
+---
+
+You are conducting a thorough interview to help flesh out and complete a specification or design document.
+
+## Your Task
+
+1. **Read the document** at `$ARGUMENTS`
+2. **Analyze it deeply** - identify gaps, ambiguities, unexplored edge cases, and areas needing clarification
+3. **Interview the user** by providing a question with some pre-determined options
+4. **Write the completed spec** back to the file when the interview is complete
+
+## Interview Guidelines
+
+### Question Quality
+- Ask **non-obvious, insightful questions** - avoid surface-level queries
+- Focus on: technical implementation details, architectural decisions, edge cases, error handling, UX implications, security considerations, performance tradeoffs, integration points, migration strategies, rollback plans
+- Each question should reveal something that would otherwise be missed
+- Challenge assumptions embedded in the document
+- Explore second and third-order consequences of design decisions
+- Use the Web Search and other tools where required to ground questions (e.g. package recommendations)
+
+### Question Strategy
+- Start by identifying the 3-5 most critical unknowns or ambiguities
+- Use the AskUserQuestion tool with well-crafted options that represent real tradeoffs
+- When appropriate, offer multiple valid approaches with their pros/cons as options
+- Don't ask about things that are already clearly specified
+- Probe deeper when answers reveal new areas of uncertainty
+
+### Topics to Explore (as relevant)
+- **Technical**: Data models, API contracts, state management, concurrency, caching, validation
+- **UX**: Error states, loading states, empty states, edge cases, accessibility, mobile considerations
+- **Operations**: Deployment, monitoring, alerting, debugging, logging, feature flags
+- **Security**: Auth, authz, input validation, rate limiting, audit trails
+- **Scale**: Performance bottlenecks, data growth, traffic spikes, graceful degradation
+- **Integration**: Dependencies, backwards compatibility, versioning, migration path
+- **Failure modes**: What happens when X fails? How do we recover? What's the blast radius?
+
+### Interview Flow
+1. Ask 2-4 questions at a time (use multiple questions in one when they're related)
+2. After each round, incorporate answers and identify follow-up questions
+3. Continue until all critical areas are addressed
+4. Signal when you believe the interview is complete, but offer to go deeper
+
+## Output
+
+When the interview is complete:
+1. Synthesize all gathered information
+2. Rewrite/expand the original document with the new details
+3. Preserve the document's original structure where sensible, but reorganize if needed
+4. Add new sections for areas that weren't originally covered
+5. Write the completed spec back to `$ARGUMENTS`
+
+Begin by reading the file and identifying your first set of deep questions.