fix(cc-config): MCP scope was inverted — expose user/local, not user/project

readMcpServers only ever parses ~/.claude.json (top-level = user scope,
projects[<root>] = the panel's "projectScoped" bucket, which the CLI itself
calls `local`). The previous user/project scope choice was the exact
inverse: `--scope project` writes .mcp.json, which readMcpServers never
reads (a successful add would vanish), and every "projectScoped" row was
actually `local`, so Remove sent a scope the CLI could never match.

- cc-mcp-cli.js: MCP_SCOPES -> ["user", "local"]; drop the local
  mutationQueue/serialize in favor of cc-plugin-cli.js's exported one, so a
  plugin install and an MCP add can't spawn `claude` concurrently; drop the
  String(v) coercion in env/header flags (a wrong-typed value is now a 400,
  not a written "[object Object]"); assertEmbeddedValue allows empty strings
  (a legitimate empty stdio arg was previously a 400).
- cc-plugin-cli.js: export `serialize` for reuse.
- CcConfig.tsx: CcMcpScope narrowed to user/local; the projectScoped bucket's
  McpCard now sends scope="local"; rows sourced from
  ~/.claude/settings.json (no CLI scope addresses them) no longer render a
  Remove button. Cleaned up i18n keys my own earlier change made dead
  (explain.mcp.addCmd/remove/removeButton) and gave the scope <select> its
  own short option labels instead of reusing full-sentence section headings.

docs/API.md

@@ -1607,7 +1607,7 @@ POST /api/cc-config/mcp/remove            Body: { name, scope? }
 
 **Plugins and marketplaces.** These five endpoints shell out to the `claude plugin …` CLI with an explicit argv array (never a shell string), pinned to the dashboard's `CLAUDE_HOME` via `CLAUDE_CONFIG_DIR`; mutating calls are serialized in-process. `source` must be `owner/repo`, an `http(s)://` URL, or an absolute path; `spec`/`key` must be `<name>` or `<name>@<marketplace>`; `scope` is `user` (default), `project` or `local`. Validation failures return 400 `EBADARG`; a missing `claude` binary returns 503 `ECLINOTFOUND`; a CLI timeout returns 504 `ETIMEOUT`; a non-zero CLI exit returns **422 `ECLIEXIT`** with the CLI's message plus `stdout`/`stderr`. Successful runs return `{ ok: true, stdout, stderr }` and broadcast `cc_config_changed`. `GET /plugins/available` proxies `claude plugin list --available --json` as `{ installed, available }`.
 
-**MCP servers.** `POST /api/cc-config/mcp/add` and `POST /api/cc-config/mcp/remove` shell out to `claude mcp add`/`claude mcp remove` the same way the plugin routes shell out to `claude plugin …` — an argv array, never a shell string, serialized in-process. `scope` is `user` or `project` only (required on add, optional on remove — omitted, the CLI removes from whichever scope the server exists in); the settings union's third value `project-local` is a different axis and is not accepted here. `add` takes either `url` (+ optional `transport: "http"|"sse"`, `headers`) for a remote server, or `command` + `args` (+ optional `env`) for a stdio server — exactly one of the two forms. Validation failures return 400 `EBADARG`; a non-zero CLI exit returns 422 `ECLIEXIT`. `GET /api/cc-config/mcp` is unaffected — it still reads `~/.claude.json` directly; the client refetches it after either call succeeds.
+**MCP servers.** `POST /api/cc-config/mcp/add` and `POST /api/cc-config/mcp/remove` shell out to `claude mcp add`/`claude mcp remove` the same way the plugin routes shell out to `claude plugin …` — an argv array, never a shell string, serialized on the SAME in-process queue `cc-plugin-cli.js` exports (`serialize`), so a plugin install and an MCP add can never spawn `claude` concurrently. `scope` is `user` or `local` only (required on add, optional on remove — omitted, the CLI removes from whichever scope the server exists in). This is the inverse of what it sounds like: `user` writes `~/.claude.json`'s top level, `local` writes its `projects[<root>]` block — the exact source `GET /mcp`'s `projectScoped` bucket reads. The CLI's own `project` scope (a separate, shared `.mcp.json`) is refused with 400: `readMcpServers` never reads that file, so a server added there would report success and never appear in the list. `add` takes either `url` (+ optional `transport: "http"|"sse"`, `headers`) for a remote server, or `command` + `args` (+ optional `env`) for a stdio server — exactly one of the two forms. Validation failures return 400 `EBADARG`; a non-zero CLI exit returns 422 `ECLIEXIT`. `GET /api/cc-config/mcp` is unaffected — it still reads `~/.claude.json` directly; the client refetches it after either call succeeds. Rows sourced from `~/.claude/settings.json` have no CLI scope that addresses them, so the panel does not offer Remove on them.
 
 `PUT /api/cc-config/keybindings` edits `~/.claude/keybindings.json` from a structured list of context groups (`{ groups: [{ context, bindings: [{ key, action }] }] }`). The server backs the file up first (under `<CLAUDE_HOME>/cc-config-backups/keybindings/`), preserves any top-level metadata (`$schema`/`$docs`), and replaces only the `bindings` array; duplicate contexts or duplicate keys within a context return `EBADCONTENT`. `keybindings.json` is replaced wholesale here because the live CLI does not rewrite it mid-session; `settings.json`, which it does, is only ever key-merged (see `PATCH /settings` above).