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/continue.md

@@ -0,0 +1,112 @@
+---
+description: Continue implementing a spec from a previous session
+argument-hint: <spec-file-path>
+---
+
+You are continuing implementation of a specification that was started in a previous session. Work autonomously until the feature is complete and tests pass.
+
+## Your Task
+
+1. **Read the spec** at `$ARGUMENTS`
+2. **Read CODE_STYLE.md** for formatting conventions
+3. **Assess current state**:
+   - Check git status for uncommitted changes
+   - Run tests to see what's passing/failing (if E2E tests exist)
+   - Review any existing implementation
+4. **Determine what remains** by comparing the spec to the current state
+5. **Plan remaining work** using TodoWrite
+6. **Continue implementing** until complete
+
+## Assessing Current State
+
+Run these commands to understand where the previous session left off:
+
+```bash
+git status                                    # See uncommitted changes
+git log --oneline -10                         # See recent commits
+npm run typecheck -w @documenso/remix         # Check for type errors
+npm run lint:fix                              # Check for linting issues
+```
+
+Review the code that's already been written to understand:
+
+- What's already implemented
+- What's partially done
+- What's not started yet
+
+## Implementation Guidelines
+
+### During Implementation
+
+- Follow CODE_STYLE.md strictly (2-space indent, double quotes, braces always, etc.)
+- Follow workspace rules for TypeScript, React, TRPC patterns, and Remix conventions
+- Mark todos complete as you finish each task
+- Commit logical chunks of work
+
+### Code Quality
+
+- No stubbed implementations
+- Handle edge cases and error conditions
+- Include descriptive error messages with context
+- Use async/await for all I/O operations
+- Use AppError class when throwing errors
+- Use Zod for validation and react-hook-form for forms
+
+### Testing
+
+**Important**: E2E tests are time-consuming. Only write tests for non-trivial functionality.
+
+- Write E2E tests in `packages/app-tests/e2e/` using Playwright
+- Test critical user flows and edge cases
+- Follow existing E2E test patterns in the codebase
+- Use descriptive test names that explain what is being tested
+- Skip tests for trivial changes (simple UI tweaks, minor refactors, etc.)
+
+## Autonomous Workflow
+
+Work continuously through these steps:
+
+1. **Implement** - Write the code for the current task
+2. **Typecheck** - Run `npm run typecheck -w @documenso/remix` to verify types
+3. **Lint** - Run `npm run lint:fix` to fix linting issues
+4. **Test** - If non-trivial, run E2E tests: `npm run test:dev -w @documenso/app-tests`
+5. **Fix** - If tests fail, fix and re-run
+6. **Repeat** - Move to next task
+
+## Stopping Conditions
+
+**Stop and report success when:**
+
+- All spec requirements are implemented
+- Typecheck passes
+- Lint passes
+- E2E tests pass (if written for non-trivial functionality)
+
+**Stop and ask for help when:**
+
+- The spec is ambiguous and you need clarification
+- You encounter a blocking issue you cannot resolve
+- You need to make a decision that significantly deviates from the spec
+- External dependencies are missing
+
+## Commands
+
+```bash
+# Type checking
+npm run typecheck -w @documenso/remix
+
+# Linting
+npm run lint:fix
+
+# E2E Tests (only for non-trivial work)
+npm run test:dev -w @documenso/app-tests              # Run E2E tests in dev mode
+npm run test-ui:dev -w @documenso/app-tests            # Run E2E tests with UI
+npm run test:e2e                                       # Run full E2E test suite
+
+# Development
+npm run dev                                            # Start dev server
+```
+
+## Begin
+
+Read the spec file and CODE_STYLE.md, assess the current implementation state, then continue where the previous session left off. Use TodoWrite to track your progress throughout.