[codex] add custom plugin registry and releases

[hrz6976]

What changed

• correct the Go module path and runtime repository metadata to use hrz6976
• add a CPA-compatible custom registry.json
• package Linux amd64 releases with the required root-level shared library and SHA-256 checksums
• publish installer assets automatically from v<version> tags
• replace Docker-based development targets with the local Go and CGO toolchains
• document CPA Manager Plus custom-registry installation
• validate the registry using CPA's own plugin-store parser

Why

This makes the plugin installable and updateable by other CLIProxyAPI deployments without waiting for inclusion in the official plugin registry.

User impact

Users can add the repository's raw registry.json URL to plugins.store-sources, then install OpenCode Go Pool through CPA Manager Plus. The first package supports Linux amd64.

Validation

• make test
• make package VERSION=0.1.0
• generated archive checksum verified with sha256sum --check
• archive contains only root-level opencode-go-pool.so
• built module path is github.com/hrz6976/cpa-plugin-opencode-go-pool
• newest required glibc symbol is GLIBC_2.34