Cannot compose `_service.sdl` from `@apollo/subgraph` on federation v2.8+: duplicate `@federation__context` / `@federation__fromContext`

## Summary

A subgraph built with `@apollo/subgraph` that links federation v2.8 or later cannot be composed from its own `_service.sdl`. Composition fails with:

```
There can be only one directive named "@federation__context".
There can be only one directive named "@federation__fromContext".
```

`@apollo/composition` accepts the same input.

## Reproduction

```js
// npm i @apollo/subgraph @theguild/federation-composition graphql
import { buildSubgraphSchema } from '@apollo/subgraph';
import { composeServices, compositionHasErrors } from '@theguild/federation-composition';
import { graphqlSync, parse } from 'graphql';

const typeDefs = parse(`
extend schema @link(url: "https://specs.apollo.dev/federation/v2.9", import: ["@key"])
type Product @key(fields: "slug") { slug: String! name: String! }
type Query { product(slug: String!): Product }
`);

// The SDL a running subgraph serves.
const sdl = graphqlSync({
schema: buildSubgraphSchema([{ typeDefs, resolvers: {} }]),
source: '{ _service { sdl } }',
}).data._service.sdl;

const result = composeServices([{ name: 's', url: 'http://localhost:4000/graphql', typeDefs: parse(sdl) }]);
console.log(compositionHasErrors(result) ? result.errors.map((e) => e.message) : 'OK');
```

Output:

```
[
'[s] There can be only one directive named "@federation__context".',
'[s] There can be only one directive named "@federation__fromContext".'
]
```

## Affected versions

Reproduced on 0.24.1 and 0.25.0.

The boundary is the linked federation version. Same repro, varying only the `@link` url:

| federation | directive definitions in `_service.sdl` | result |
| ---------- | --------------------------------------- | ------ |
| v2.0 | 10 | OK |
| v2.3 | 12 | OK |
| v2.5 | 14 | OK |
| v2.7 | 15 | OK |
| v2.8 | 17 | FAIL |
| v2.9 | 19 | FAIL |

v2.8 is where `@context` and `@fromContext` entered the spec.

## Comparison with @apollo/composition

Same SDL, same call shape:

```
@apollo/composition 2.14.4 : OK
@theguild/federation-composition 0.25.0 : FAIL (2)
```

## Why it matters

`_service.sdl` is what a running subgraph serves, so it is the SDL you get by introspecting a live service instead of reading an authored file. Anything composing from a live subgraph hits this: `hive dev --url ...`, registry ingestion via introspection, and CI gates that compose a service's emitted SDL.

Note that only these two collide. The other 17 definitions `_service.sdl` prints are tolerated, which is what makes this look like an oversight rather than a deliberate rejection of subgraph-supplied definitions.

## Workaround

Strip directive definitions from the composer's input before calling `composeServices`. Dropping only the two colliding definitions is enough. In our case, dropping the whole `link` / `key` / `federation__` family produced a byte-identical supergraph, so the coarser filter is safe too.

## Possibly related

#85 tracks `@context` and `@fromContext` composition support and may share a root cause.

Run the supplied reproduction using buildSubgraphSchema, graphqlSync, and composeServices, then compare the emitted SDL and composition behavior across federation v2.7 and v2.8. Trace how composeServices handles the @federation__context and @federation__fromContext definitions. Done means composing the emitted _service.sdl succeeds for federation v2.8 and later without duplicate-directive errors.

Written by the indexing model from the issue text.