docs(go): add Claude Code skill documentation

Documents the core CLI skill for Claude Code:
- Installation methods (project-based and global)
- Usage patterns (automatic and manual invocation)
- What Claude learns (testing, building, multi-repo)
- Command reference overview
- Customisation options

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/packages/go/cmd/skill.md

@@ -0,0 +1,125 @@
+# Claude Code Skill
+
+The `core` CLI includes a Claude Code skill that helps Claude use the correct commands when working in host-uk repositories.
+
+## What It Does
+
+The skill provides Claude with:
+- Command quick reference for all `core` commands
+- Decision tree for choosing the right command
+- Common mistakes to avoid
+- Best practices for testing, building, and releasing
+
+## Installation
+
+### Automatic (Project-Based)
+
+When working in any host-uk repository that includes `.claude/skills/core/`, Claude automatically discovers and uses the skill.
+
+### Global Install
+
+Install the skill globally so it works in any project:
+
+```bash
+# If you have the repo cloned
+cd /path/to/core
+./.claude/skills/core/install.sh
+
+# Or via curl
+curl -fsSL https://raw.githubusercontent.com/host-uk/core/main/.claude/skills/core/install.sh | bash
+```
+
+This copies the skill to `~/.claude/skills/core/`.
+
+## Usage
+
+### Automatic Invocation
+
+Claude automatically uses the skill when:
+- Running tests in a Go project
+- Building or releasing
+- Working across multiple repos
+- Checking CI status or issues
+
+### Manual Invocation
+
+Type `/core` in Claude Code to see the full command reference.
+
+## What Claude Learns
+
+### Testing
+
+```
+Wrong: go test ./...
+Right: core test
+
+Why: core test sets MACOSX_DEPLOYMENT_TARGET, filters linker warnings,
+     and provides colour-coded coverage output.
+```
+
+### Building
+
+```
+Wrong: go build
+Right: core build
+
+Why: core build handles cross-compilation, code signing, archiving,
+     and checksums automatically.
+```
+
+### Multi-Repo Workflows
+
+```
+Wrong: cd into each repo, run git status
+Right: core health
+
+Why: Aggregated view across all repos in one command.
+```
+
+## Command Reference
+
+The skill includes documentation for:
+
+| Category | Commands |
+|----------|----------|
+| Testing | `core test`, `core test --coverage`, `core test --json` |
+| Building | `core build`, `core build --targets`, `core build --ci` |
+| Releasing | `core release`, `core sdk` |
+| Multi-Repo | `core health`, `core work`, `core commit`, `core push`, `core pull` |
+| GitHub | `core issues`, `core reviews`, `core ci` |
+| Environment | `core doctor`, `core setup`, `core search`, `core install` |
+| PHP | `core php dev`, `core php artisan` |
+| Containers | `core run`, `core ps`, `core stop`, `core logs`, `core exec` |
+| Docs | `core docs list`, `core docs sync` |
+
+## Customisation
+
+The skill is a markdown file at `.claude/skills/core/SKILL.md`. You can:
+
+1. **Fork and modify** - Copy to your own repo's `.claude/skills/` and customise
+2. **Extend** - Add project-specific commands or workflows
+3. **Override** - Project skills take precedence over global skills
+
+## Troubleshooting
+
+### Skill Not Loading
+
+Check if the skill exists:
+```bash
+ls ~/.claude/skills/core/SKILL.md
+# or
+ls .claude/skills/core/SKILL.md
+```
+
+### Reinstall
+
+```bash
+rm -rf ~/.claude/skills/core
+curl -fsSL https://raw.githubusercontent.com/host-uk/core/main/.claude/skills/core/install.sh | bash
+```
+
+## See Also
+
+- [test command](test.md) - Run tests with coverage
+- [build command](build.md) - Build projects
+- [work command](work.md) - Multi-repo operations

docs/packages/go/index.md

@@ -61,6 +61,12 @@ curl -fsSL https://github.com/host-uk/core/releases/latest/download/core-$(uname
 |---------|-------------|
 | [`core docs`](cmd/docs.md) | Documentation management |
 
+### Integrations
+
+| Integration | Description |
+|-------------|-------------|
+| [Claude Code Skill](cmd/skill.md) | AI assistant integration for command guidance |
+
 ## Quick Start
 
 ```bash
@@ -111,6 +117,7 @@ Core uses `.core/` directory for project configuration:
 - [Setup](cmd/setup.md) - Clone repos from registry
 - [Work](cmd/work.md) - Multi-repo operations
 - [Docs](cmd/docs.md) - Documentation management
+- [Claude Code Skill](cmd/skill.md) - AI assistant integration
 
 ### Reference
 - [Configuration](configuration.md) - All config options