Add OpenCode compatibility installer for plugin workflows

[marthendalnunes]

Summary

• Add a first-party OpenCode installer (scripts/install_opencode_skills.sh) that installs plugin skills and commands, preserving the same command-first plugin UX (command -> skill) used in Claude.
• Default to simple no-clone installation from GitHub archives (copy mode), with local symlink mode for contributors; support filters (--bundle, --plugin, --skill, --command) and component scoping (--skills-only, --commands-only).
• Add compatibility guardrails: validate command-to-skill mapping, skip known Claude-only command flows by default, and add scripts/validate_opencode_compat.py plus CI smoke checks.

Docs

• Add OpenCode compatibility guide at docs/opencode.md.
• Update README.md with OpenCode install and command usage examples.

Validation

• python3 scripts/validate_opencode_compat.py
• python3 .github/scripts/validate_plugin_metadata.py
• bash scripts/install_opencode_skills.sh --source local --bundle smart-contracts --dry-run --target /tmp/opencode-skills-smoke --commands-target /tmp/opencode-commands-smoke

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