[Schema][Server] Add Skills extension (io.modelcontextprotocol/skills) support

[wachterjohannes]

Summary

Implements the MCP Skills extension (SEP-2640): a way for servers to ship skills — multi-step workflow instructions that tell an agent how to orchestrate tools to reach a goal. Skills are served through the existing Resources primitive with zero protocol changes, mirroring the existing MCP Apps extension (io.modelcontextprotocol/ui).

A server can expose a whole directory of skills in one line:

$server = Server::builder()
    ->setServerInfo('My Server', '1.0.0')
    ->addSkillsFromDirectory(__DIR__.'/skills')
    ->build();

This registers each SKILL.md (and its supporting files) as skill:// resources, derives name/description from the YAML frontmatter, and serves a skill://index.json discovery index.

What's included

• Schema/Extension/Skills/McpSkills — extension marker (EXTENSION_ID, MIME_TYPE, URI_SCHEME, ENTRY_POINT, DISCOVERY_URI, META_PREFIX).
• DTOs — SkillType (enum), SkillDiscoveryEntry, SkillDiscoveryIndex, SkillMetadata, following the existing Apps DTO conventions (readonly, fromArray, omit-null jsonSerialize).
• Server/Skill/FrontmatterParser — splits SKILL.md into YAML frontmatter + body (handles BOM/CRLF; rejects non-mapping frontmatter).
• Server/Skill/SkillProvider — walks a directory, registers each skill and its supporting files as skill:// resources, enforces the spec's name↔final-path-segment rule, sanitizes schema-valid resource names (URIs keep real paths), guesses MIME types, guards against path traversal, and serves the discovery index.
• Server/Builder::addSkillsFromDirectory() — convenience that auto-enables the extension.

Notable fix

ServerCapabilities::jsonSerialize() only cast the outer extensions map, so an extension with an empty payload (like Skills) serialized to [] instead of {}. The Apps extension never hit this because its payload is non-empty. Empty inner payloads are now coerced to {}.

Dependency

Adds symfony/yaml (^5.4 || ^6.4 || ^7.3 || ^8.0) for frontmatter parsing — the feature is non-functional without it.

Tests & docs

• Unit tests for the extension, DTOs, FrontmatterParser, and SkillProvider (with fixtures), plus a serialization regression test for the {} fix.
• Inspector stdio snapshot test (resources/list, resources/read of a SKILL.md, a supporting file, and the discovery index).
• Example server at examples/server/skills/ (flat, nested, and supporting-file URIs).
• Docs in docs/extensions.md and docs/examples.md.

Verified: phpstan (level 6) clean, php-cs-fixer clean, full unit suite green (792 tests), inspector skills test green.

Deferred / follow-up

The discovery schema supports a mcp-resource-template skill type for parameterized skill namespaces. The SkillType enum already includes the value for forward-compat, but the directory provider currently emits only skill-md entries; template-backed skills are left as a follow-up.