Add in-memory GitHub plugin loading

[pbdeuchler]

Summary

• Move loaded resource structs and disk plugin/skill loaders into halter-config, keeping halter as the resource compiler/facade and preserving its existing re-exports.
• Add halter_config::github behind remote-plugins for in-memory GitHub tarball loading of plugin directories and marketplaces.
• Use Codex manifests first (.codex-plugin/plugin.json, .agents/plugins/marketplace.json) with Claude fallbacks, and fail closed for remote sources when neither exists.
• Skip command hooks from remote in-memory plugins with a hook warning instead of materializing executable files on disk.

Closes #147.

Tests

• cargo test -p halter-config
• cargo test -p halter-config --features remote-plugins
• cargo test -p halter --features remote-plugins
• cargo check --workspace

[pbdeuchler]

Remove .halter-plugin functionality, we will use codex/claude/agent patterns