docs(skills): Add OpenCode knowledge base and skills documentation

[thoroc]

Summary

OpenCode skills/knowledge base documentation and using-opencode-core guide.

Changes

Add 8 skill documentation files covering core development topics:

• bun-typescript-development: TypeScript + Bun best practices
• extending-nx-plugins: Nx plugin development patterns
• git-commit: Git workflow and commit conventions
• nx-executors: Custom Nx executor patterns
• nx-generators: Nx generator development
• opencode-plugin-dev: OpenCode plugin development
• opencode-plugin-development: Extended plugin patterns
• opencode-plugin-ui: TUI integration patterns

Additional documentation:

• docs/using-opencode-core.md - User guide for the core plugin

Dependencies

• Requires: PR feat(infra): Add Biome linting/formatting infrastructure #21 (Biome formatting)
• Optional: PR feat(core): Add opencode-core-plugin with 6 specialized skills #26 (core plugin) - can reference core plugin features

Blocks

• None

Testing

• Markdown linting: bun run lint:md

[github-actions]

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• 9 changed files
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[Copilot]

Pull request overview

This PR adds a set of OpenCode skills and a usage guide for the @pantheon-org/opencode-core notification library, targeting plugin and Nx development workflows. The changes primarily expand developer-facing documentation and skill metadata in .opencode/skills plus a new guide under docs/.

Changes:

• Add docs/using-opencode-core.md with examples and patterns for using the opencode-core notification helper in plugins.
• Introduce multiple OpenCode skills for Bun + TypeScript standards, Nx generators/executors/plugins, Git commit workflow, and OpenCode plugin UI & development patterns.
• Provide detailed examples and best practices for TUI integration, plugin hooks, and Nx plugin extensibility.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/using-opencode-core.md	Adds a usage guide for the @pantheon-org/opencode-core notification helper API with plugin examples and migration guidance from direct TUI calls.
.opencode/skills/opencode-plugin-ui/SKILL.md	Documents a skill for OpenCode plugin UI integration via client.tui.showToast, client.session.prompt(noReply), and client.tui.appendPrompt, including best practices and a full example plugin.
.opencode/skills/opencode-plugin-development/SKILL.md	Provides a comprehensive skill covering OpenCode plugin architecture, hooks, auth providers, events, shell integration, testing, and best practices.
.opencode/skills/opencode-plugin-dev/SKILL.md	Adds a more guided, narrative skill for OpenCode plugin development, including context API, hooks, shell usage, testing, debugging, and recipes.
.opencode/skills/nx-generators/SKILL.md	Introduces a skill on writing Nx generators, including Tree API usage, schema definitions, composing generators, templating, testing, and troubleshooting.
.opencode/skills/nx-executors/SKILL.md	Adds a skill for custom Nx executors, covering schema and implementation structure, package-based executor references, ExecutorContext, testing, and migration from relative paths.
.opencode/skills/git-commit/SKILL.md	Documents a Git commit workflow skill that enforces safe rebasing, remote sync, and mandatory hook execution before committing.
.opencode/skills/extending-nx-plugins/SKILL.md	Provides a skill summarizing how to extend Nx via plugins (generators, inferred tasks, presets, migrations) with examples and best practices.
.opencode/skills/bun-typescript-development/SKILL.md	Defines Bun + TypeScript development standards for this workspace, including TS config, module strategy, Biome usage, testing patterns, and file naming/type patterns.

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[github-actions]

⚠️ Large PR Warning

This PR is quite large with:

• 9 changed files
• 3511 added lines
• 0 deleted lines

Consider breaking it down into smaller, focused PRs for easier review.