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

[steve-calvert-glean]

First of three PRs adding Agent Plugins support. This one builds the MCP piece and changes no output for configs that already work.

Why

Authored MCP config was copied verbatim into every target. The README said pluginpack "translates the configuration into each target's native MCP layout", but only the file path changed. Each client expects different variable names (${CLAUDE_PLUGIN_ROOT}, ${CURSOR_PLUGIN_ROOT}, ${PLUGIN_ROOT}), so authors forked mcp/config.json per target. gleanwork/agent-plugins keeps overrides/codex/glean/mcp/config.json for exactly this. Agent Plugins output needs $schema, an explicit type, and spec-valid fields, so this has to be fixed before that format can be emitted.

What

• src/mcp.ts: renderMcpConfig(servers, dialect, context) holds every client's MCP dialect: variable names, transport labels, and the Agent Plugins 1.0 rules.
• Each target sets one mcpDialect; the engine renders through the module.

Dialect	Root / data variables	Transport
claude	${CLAUDE_PLUGIN_ROOT} / ${CLAUDE_PLUGIN_DATA}	streamable-http → http
cursor	${CURSOR_PLUGIN_ROOT} / none (build error)	type dropped for stdio/HTTP (Cursor infers it)
copilot	${PLUGIN_ROOT} / as authored	streamable-http → http
verbatim	as authored (codex, antigravity for now)	as authored
agent-plugins	${PLUGIN_ROOT} / ${PLUGIN_DATA} + $schema	explicit type; spec §7.2 / §9 enforced

The agent-plugins dialect turns anything a conforming client would silently skip into a build error: a shell string as command, ${API_TOKEN} in env (left unexpanded), plain http to a remote host, reserved env names, or a cwd that escapes the plugin. No target uses this dialect yet; the next PR switches codex to it.

Also adds CONTEXT.md (glossary) and docs/adr/0001 (codex emits Agent Plugins packages by default, with no dual layout).

Behavior change

• Cursor: ${PLUGIN_DATA} / ${CLAUDE_PLUGIN_DATA} in MCP config is now a build error. Cursor has no plugin-data variable, so that config never worked there.
• Claude/Cursor: a ./bin/server command becomes ${<root variable>}/bin/server.
• Config already in a target's own dialect is emitted unchanged.

Verification

• npm run check passes (239 tests; 40 new in tests/mcp.test.ts, plus one build-level wiring test).
• Built gleanwork/agent-plugins with released 0.11.0 and with this branch (temp copies): all 191 output files are byte-identical, including the claude, cursor, and codex .mcp.json files.
• Rendering Glean's shared config in the agent-plugins dialect gives valid Agent Plugins mcp.json. The current codex override is rejected because its cwd: "." isn't a valid Agent Plugins path.

Next

• (b) Agent Plugins package module, a codex format option (default agent-plugins), a standalone agent-plugins target, and pinned 1.0.0 schemas. Release as 0.12.0.
• (c) agent-plugins: delete the codex MCP override, convert the shared config, and regenerate codex-plugins.