Parametrize skill install + auto-install during init

[renardeinside]

Summary

• Add --path flag to apx skill install (default: .claude/skills/apx) so different assistants can use different skill directories
• Extract install_skills_to() as a reusable function and call it during apx init for any addon with a skill_path in its manifest
• Rename CLAUDE.md → CLAUDE.md.jinja2 with {{app_name}} template variables so generated projects get concrete paths
• Add skill_path to all assistant addon manifests (codex uses .agents/skills/apx/, others use .claude/skills/apx/)
• Update CLI, Claude Code, and MCP docs to reflect the new --path option and auto-install behavior

Test plan

• cargo build compiles
• cargo test -p apx-cli — all 28 tests pass
• apx skill install — installs to .claude/skills/apx/ by default
• apx skill install --path .agents/skills/apx — installs to codex path
• apx init --addons=ui,claude — has CLAUDE.md (rendered), .claude/skills/apx/, .mcp.json, hooks/
• apx init --addons=ui,cursor — has .claude/skills/apx/, .mcp.json, hooks/
• apx init --addons=ui,codex — has .agents/skills/apx/, .mcp.json, hooks/

🤖 Generated with Claude Code