[FEAT] Bootstrap four-opencode-git-tools plugin

[four-bytes-robby]

Purpose

Bootstrap the plugin skeleton from the opencode-plugins template with:

• 5 tool stubs: curse_score, bus_factor, implicit_coupling, ownership, blast_radius
• Build config, CI, tests
• AGENTS.md and README.md

Scope

• Plugin skeleton created
• 5 tool stubs registered
• Tests (2) verify module loads + 5 tools
• Implement curse_score tool
• Implement bus_factor tool
• Implement implicit_coupling tool
• Implement ownership tool
• Implement blast_radius tool

Closes: bootstrap task