Add skills system with Claude Code compatibility (#171)

* Add skills system with Claude Code compatibility

* consolidate skills into single module, merge loaders, add <available_skills> XML tags

* add Codex CLI skills compatibility, skip hidden/symlinks

packages/coding-agent/docs/skills.md

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+# Skills
+
+Skills are instruction files that the agent loads on-demand for specific tasks.
+
+## Skill Locations
+
+Skills are discovered from these locations (in order of priority, later wins on name collision):
+
+1. `~/.codex/skills/**/SKILL.md` (Codex CLI user skills, recursive)
+2. `~/.claude/skills/*/SKILL.md` (Claude Code user skills)
+3. `<cwd>/.claude/skills/*/SKILL.md` (Claude Code project skills)
+4. `~/.pi/agent/skills/**/*.md` (Pi user skills, recursive)
+5. `<cwd>/.pi/skills/**/*.md` (Pi project skills, recursive)
+
+Skill names and descriptions are listed in the system prompt. When a task matches a skill's description, the agent uses the `read` tool to load it.
+
+## Creating Skills
+
+A skill is a markdown file with YAML frontmatter containing a `description` field:
+
+```markdown
+---
+description: Extract text and tables from PDF files
+---
+
+# PDF Processing Instructions
+
+1. Use `pdftotext` to extract plain text
+2. For tables, use `tabula-py` or similar
+3. Always verify extraction quality
+
+Scripts are in: {baseDir}/scripts/
+```
+
+### Frontmatter Fields
+
+| Field | Required | Description |
+|-------|----------|-------------|
+| `description` | Yes | Short description for skill selection |
+| `name` | No | Override skill name (defaults to filename or directory name) |
+
+The parser only supports single-line `key: value` syntax. Multiline YAML blocks are not supported.
+
+### Variables
+
+`{baseDir}` is replaced with the directory containing the skill file. Use it to reference bundled scripts or resources.
+
+### Subdirectories (Pi Skills)
+
+Pi skills in subdirectories use colon-separated names:
+- `~/.pi/agent/skills/db/migrate.md` → `db:migrate`
+- `<cwd>/.pi/skills/aws/s3/upload.md` → `aws:s3:upload`
+
+## Claude Code Compatibility
+
+Pi reads Claude Code skills from `~/.claude/skills/*/SKILL.md`. The `allowed-tools` and `model` frontmatter fields are ignored since Pi cannot enforce them.
+
+## Codex CLI Compatibility
+
+Pi reads Codex CLI skills from `~/.codex/skills/`. Unlike Claude Code skills (one level deep), Codex skills are scanned recursively, matching Codex CLI's behavior. Hidden files/directories (starting with `.`) and symlinks are skipped.
+
+## Disabling Skills
+
+CLI flag:
+```bash
+pi --no-skills
+```
+
+Or in `~/.pi/agent/settings.json`:
+```json
+{
+  "skills": {
+    "enabled": false
+  }
+}
+```
+
+## Example
+
+```markdown
+---
+description: Perform code review with security and performance analysis
+---
+
+# Code Review
+
+Analyze:
+
+## Security
+- Input validation
+- SQL injection
+- XSS vulnerabilities
+
+## Performance
+- Algorithm complexity
+- Memory usage
+- Query efficiency
+```