Format crates under crates/formats/, each described in Rust instead of plugin.toml #442

After #437, Forge loads no WebAssembly plugins. Every package format is a crate the server links in. Two things are left over from when a format could also ship as a component that an operator installs:

• plugins/ is a second top-level directory of workspace crates. It used to set apart the crates that build for wasm32-wasip2. Now it only adds an entry to the CI change filter (.github/scripts/ci-changes.sh), to CodeQL's filter (.github/workflows/codeql.yml), to .gitignore and to .dockerignore, and it backs a rule in AGENTS.md that keeps formats out of crates/.
• plugin.toml was the manifest an installed component carried next to its .wasm. A built-in format still reads it at startup:
  • the crate embeds it as text (forge_plugin_api::plugin_toml!, an include_str!);
  • the server parses it with toml and expects the result ("plugins/nuget/plugin.toml is valid");
  • the id is leaked to get a &'static str;
  • its version must equal the crate's, and a test in each crate checks that;
  • its api (the contract version) can no longer disagree with the host, because both compile together.

Target

crates/formats/
├── README.md   # was plugins/README.md
├── nuget/      # forge-format-nuget
├── raw/        # forge-format-raw (#431)
└── docker/     # was crates/format-oci (forge-oci)

Each format describes itself in Rust, next to its impl FormatPlugin: id, name, description, kinds, assets and storage. There is no plugin.toml and no plugins/.

Steps, one pull request each

1. Move the format crates from plugins/<id> to crates/formats/<id> #443: move plugins/<id> to crates/formats/<id>, and change the AGENTS.md rule to match.
2. Describe each format in Rust and remove plugin.toml #444: replace plugin.toml with a descriptor in Rust.
3. Move the Docker / OCI crate from crates/format-oci to crates/formats/docker #445: move crates/format-oci to crates/formats/docker.

#443 and #444 are independent of each other but touch the same files, so land one before starting the other. #445 follows #443, which sets the new rule in AGENTS.md.

Before starting

These open changes rewrite or add files under plugins/ or in plugin.toml. Work started before they merge will conflict with each of them.

• Remove WebAssembly format plugins and Wasmtime: built-in plugins only #440 (Remove WebAssembly format plugins and wasmtime: built-in plugins only #437) removes the WebAssembly side, plugins/helm, and the component keys of plugin.toml.
• Raw repositories as a built-in format plugin over a host file store #441 (Raw repositories as a format plugin: a file store in the plugin contract #431) adds plugins/raw and a storage key to plugin.toml.
• Serve Helm charts from the OCI registry only, with a chart page in the web UI #435 (Helm charts: classic chart repositories and OCI charts #43) changes plugins/helm, which Remove WebAssembly format plugins and Wasmtime: built-in plugins only #440 then deletes.
• Serve every repository at /repository/{name}/, with no format in the URL (breaking) #438 (no pull request yet) removes mount from plugin.toml.

Out of scope

• The plugin vocabulary. This covers the crates forge-plugin and forge-plugin-api, the trait FormatPlugin and docs/format-plugins.md. Whether to rename them is a separate decision. This series changes only where the crates live and how a format describes itself.
• Docker / OCI as a FormatPlugin. Move the Docker / OCI crate from crates/format-oci to crates/formats/docker #445 moves the crate as it is.