feat: render MCP config into each target's MCP dialect

CONTEXT.md

@@ -0,0 +1,50 @@
+# pluginpack
+
+pluginpack compiles one authored source of agent plugins into the plugin
+layouts and catalogs each AI client expects.
+
+## Language
+
+### Authoring
+
+**Source**:
+A directory holding one plugin's shared, client-neutral content.
+_Avoid_: shared plugin, source plugin (that term names the legacy 0.10 discovered-plugin model)
+
+**Overrides**:
+A per-target directory applied after a source, adding or replacing files for one target.
+_Avoid_: target overlay, targets/ replacements
+
+**Content kind**:
+A selectable category of source content: a component directory, `static`, or `mcp`.
+_Avoid_: component (when `static` or `mcp` is meant)
+
+### Output
+
+**Target**:
+One output configuration that pluginpack builds, named for the client or format it serves.
+_Avoid_: host, adapter (in config), platform
+
+**Package**:
+One plugin directory laid out to the Agent Plugins specification: `plugin.json`, `skills/`, `mcp.json`, and any extension directories.
+_Avoid_: portable plugin, AP plugin
+
+**Client profile**:
+A client's additions to a package: its extension namespace, the manifest data under it, and the files in its extension directory.
+_Avoid_: overlay, flavor
+
+**Extension namespace**:
+The reverse-domain key a client owns inside a package, such as `com.openai`.
+_Avoid_: vendor key, client key
+
+**Marketplace**:
+A client's catalog file listing installable plugins, such as `.agents/plugins/marketplace.json`. Not part of a package.
+_Avoid_: registry, index, catalog (as a term)
+
+**MCP dialect**:
+The plugin-root and plugin-data variable names and transport labels one client's MCP config expects.
+_Avoid_: MCP format, MCP flavor
+
+**Artifact**:
+The in-memory map of every file one target build emits, plus which paths pluginpack manages.
+_Avoid_: output bundle

README.md

@@ -342,17 +342,27 @@ shared/acme/mcp/
 }
 ```
 
-Pluginpack translates the configuration into each target's native MCP layout.
 The MCP source and tests remain in `mcp/`; only declared shipping files are
 emitted. Add `"mcp"` to `exclude` to omit the complete capability for a target.
 
-| Target        | How MCP is wired                                         |
-| ------------- | -------------------------------------------------------- |
-| `claude`      | ships `.mcp.json` at the plugin root (auto-discovered)   |
-| `cursor`      | ships `.mcp.json`, referenced from `plugin.json`         |
-| `codex`       | ships `.mcp.json`, referenced from `plugin.json`         |
-| `copilot`     | ships `.mcp.json`, referenced from the marketplace entry |
-| `antigravity` | writes `mcp_config.json` beside `plugin.json`            |
+Author `config.json` once, in either the
+[Agent Plugins](https://agent-plugins.org/specification) form (an explicit
+`type` per server, `${PLUGIN_ROOT}` / `${PLUGIN_DATA}`) or the Claude-style
+form (`type` optional, `${CLAUDE_PLUGIN_ROOT}`). Pluginpack renders it into
+each target's MCP dialect, so you don't need a per-target `config.json`
+override just to change a variable name:
+
+| Target        | File                                         | Plugin root / data variables                      | Transport labels                                       |
+| ------------- | -------------------------------------------- | ------------------------------------------------- | ------------------------------------------------------ |
+| `claude`      | `.mcp.json` at the plugin root               | `${CLAUDE_PLUGIN_ROOT}` / `${CLAUDE_PLUGIN_DATA}` | `streamable-http` becomes `http`                       |
+| `cursor`      | `.mcp.json`, referenced from `plugin.json`   | `${CURSOR_PLUGIN_ROOT}` / none (a build error)    | `type` dropped for stdio and HTTP (Cursor infers them) |
+| `copilot`     | `.mcp.json`, referenced from the marketplace | `${PLUGIN_ROOT}` / as authored                    | `streamable-http` becomes `http`                       |
+| `codex`       | `.mcp.json`, referenced from `plugin.json`   | as authored                                       | as authored                                            |
+| `antigravity` | `mcp_config.json` beside `plugin.json`       | as authored                                       | as authored                                            |
+
+A `./bin/server` command becomes `${<root variable>}/bin/server` for targets
+whose clients don't resolve plugin-relative commands. Config already written in
+a target's own dialect is emitted unchanged.
 
 ## Legacy Additional Plugin-Root Files
 

docs/adr/0001-codex-emits-agent-plugins-packages.md

@@ -0,0 +1,18 @@
+---
+status: accepted
+---
+
+# Codex emits Agent Plugins packages by default, with no dual layout
+
+OpenAI's packaging docs make the Agent Plugins root `plugin.json` (OpenAI
+settings under `extensions.com.openai`) the preferred format and keep
+`.codex-plugin/plugin.json` only as a fallback. From 0.12.0 the `codex` target
+therefore emits Agent Plugins packages by default. A `format: "legacy"` option
+keeps the old layout for one release window, for users on Codex older than
+v0.146.
+
+We deliberately do not emit both layouts in one build. A dual layout would need
+two MCP files in different shapes (`mcp.json` and `.mcp.json`) and a
+`.codex-plugin` overlay that current Codex ignores whenever
+`extensions.com.openai` is present. That doubles what can drift, only to serve
+older clients.