feat: support the Skills extension (SEP-2640) with digest verification

Adds detection, enumeration, and conformance checking for the Skills
extension (`io.modelcontextprotocol/skills`) — phases 1 and 2 of #2234,
which is everything its Acceptance list names.

Skills is a *server*-declared extension, read off the connecting server's
`capabilities.extensions`, so it is deliberately absent from
`ADVERTISABLE_EXTENSIONS` — that registry is what the Inspector advertises
and the user toggles, and an entry there would be a meaningless toggle.

`skills/list` and `skills/get` are consumer-owned extension methods that
neither era codec defines, so the SDK's era gate skips them and they go out
as ordinary `client.request` calls with explicit result schemas. The
raw-wire channel modern `tasks/*` needs is not used, and the Skills tab is
not era-gated: a legacy-era server that declares the extension is serving it.

The checks are the point, not the list view. `core/mcp/skills.ts` reports
each obligation SEP-2640 states — the name/path invariant, the digest
format, the 512-entry and 16 MiB limits, and `resources: "dynamic"`, which
means integrity cannot be verified at all. Digest verification hashes the
fetched bytes with WebCrypto and returns a mismatch with both digests
attached rather than throwing, because showing a mismatch loudly is the
whole value proposition.

Files are fetched on demand: SEP-2640 is explicit that a `resources/read`
of a `SKILL.md` is not a load and confers no standing, so none of the SEP's
host machinery is implemented.

The `skills-http` fixture serves four skills over two pages, three of them
deliberately non-conforming — without those the verification code is
untestable.

Phase 3 (CLI, TUI, `resources/directory/read`, paged mode) is #2248.

Closes #2234

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018kfrH3uRNUM4JAk1reKtyY
Signed-off-by: cliffhall <cliff@futurescale.com>

test-servers/src/load-config.ts

@@ -67,6 +67,9 @@ export interface ConfigFile {
    * and wire its handlers + `modern_task` / `modern_input_task` tools. Pair with
    * `transport.modern`. */
   tasksExtension?: boolean;
+  /** Advertise the Skills extension (SEP-2640) and serve its fixture skills.
+   * `directoryRead` advertises the `resources/directory/read` sub-option. */
+  skills?: { directoryRead?: boolean };
   /** Advertise the MCP Apps `io.modelcontextprotocol/ui` extension with the nested
    * `elicitation` setting — the server half of app-rendered form elicitation
    * (#1854). Pair with the `app_choose_option` tool + `choose_option_app` resource. */