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/commit.md

@@ -0,0 +1,80 @@
+---
+description: Add and commit changes using conventional commits
+allowed-tools: Bash, Read, Glob, Grep
+---
+
+Create a git commit for the current changes using the Conventional Commits standard.
+
+## Process
+
+1. **Analyze the changes** by running:
+   - `git status` to see all modified/untracked files
+   - `git diff` to see unstaged changes
+   - `git diff --staged` to see already-staged changes
+   - `git log --oneline -5` to see recent commit style
+
+2. **Stage appropriate files**:
+   - Stage all related changes with `git add`
+   - Do NOT stage files that appear to contain secrets (.env, credentials, API keys, tokens)
+   - If you detect potential secrets, warn the user and skip those files
+
+3. **Determine the commit type** based on the changes:
+   - `feat`: New feature or capability
+   - `fix`: Bug fix
+   - `docs`: Documentation only
+   - `style`: Formatting, whitespace (not CSS)
+   - `refactor`: Code restructuring without behavior change
+   - `perf`: Performance improvement
+   - `test`: Adding or updating tests
+   - `build`: Build system or dependencies
+   - `ci`: CI/CD configuration
+   - `chore`: Maintenance tasks, tooling, config
+
+   NOTE: Do not use a scope for commits
+
+4. **Write the commit message**:
+   - **Subject line**: `<type>: <description>`
+     - Use imperative mood ("add" not "added")
+     - Lowercase, no period at end
+     - Max 50 characters if possible, 72 hard limit
+   - **Body** (if needed): Explain _why_, not _what_
+     - Wrap at 72 characters
+     - Separate from subject with blank line
+
+## Commit Format
+
+```
+<type>[scope]: <subject>
+
+[optional body explaining WHY this change was made]
+```
+
+## Examples
+
+Simple change:
+
+```
+fix: handle empty input in parser without throwing
+```
+
+With body:
+
+```
+feat: add streaming response support
+
+Large responses were causing memory issues in production.
+Streaming allows processing chunks incrementally.
+```
+
+## Rules
+
+- NEVER commit files that may contain secrets
+- NEVER use `git commit --amend` unless the user explicitly requests it
+- NEVER use `--no-verify` to skip hooks
+- If the pre-commit hook fails, fix the issues and create a NEW commit
+- If there are no changes to commit, inform the user and stop
+- Use a HEREDOC to pass the commit message to ensure proper formatting
+
+## Execute
+
+Run the git commands to analyze, stage, and commit the changes now.