feat(skills): introduce v1 installable skills framework

[anshitraj]

Summary

This PR introduces a minimal v1 installable skills framework for OmniClaw.

Motivation

OmniClaw already supports instruction-based skills under .agents/skills (for example omniclaw-cli/SKILL.md). Those are primarily used as guidance for AI agents.

This contribution adds a lightweight runtime layer that allows skills to be packaged with metadata and discovered automatically from disk while keeping the existing structure unchanged.

What this PR adds

• Manifest-based skill definition (skill.json)
• Local skill discovery from .agents/skills
• Validation of skill manifests
• In-memory skill registry
• Enable/disable support in registry
• Example skill (hello-world)
• Tests and documentation

Design goals

• Fully additive and backward-compatible
• No changes to payment, nanopayment, wallet, guard, trust, or MCP code
• Minimal implementation suitable for open-source contribution
• Keeps .agents/skills instruction files untouched

Not included in v1

• Remote skill registry
• CLI skill management commands
• Runtime permission enforcement
• Persistent skill state
• Marketplace or distribution layer

Why this helps

This lays the groundwork for modular skill-based extensions so developers building AI agents on OmniClaw can package reusable payment behaviors without modifying core SDK logic.

Closes #29

[abiorh-claw]

@anshitraj you need to add at least one reviewer