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>

core/mcp/inspectorClientProtocol.ts

@@ -40,6 +40,8 @@ import type {
 import type { JsonValue } from "../json/jsonUtils.js";
 import type { MalformedListItem } from "./listSalvage.js";
 import type { InspectorClientEventTarget } from "./inspectorClientEventTarget.js";
+import type { SkillEntry } from "./skillsSchemas.js";
+import type { SkillsExtensionSupport } from "./skills.js";
 import type { SamplingCreateMessage } from "./samplingCreateMessage.js";
 import type { ElicitationCreateMessage } from "./elicitationCreateMessage.js";
 
@@ -110,6 +112,17 @@ export interface InspectorClientProtocol extends InspectorClientEventTarget {
    * and the modern task store's poll-based refresh. */
   isTasksExtensionNegotiated(): boolean;
 
+  /** The Skills extension (SEP-2640) the server declared, or `undefined`.
+   * Gates the Skills tab and the managed skills store (#2234). */
+  getSkillsExtension(): SkillsExtensionSupport | undefined;
+  /** One page of `skills/list`; the managed skills store walks the cursor. */
+  listSkills(
+    cursor?: string,
+    metadata?: RequestMetadata,
+  ): Promise<{ skills: SkillEntry[]; nextCursor?: string }>;
+  /** One skill entry by URI (`skills/get`). */
+  getSkill(uri: string, metadata?: RequestMetadata): Promise<SkillEntry>;
+
   /**
    * Mark the response that most recently answered `method` as rejected by the
    * client, so its Protocol entry shows the reason instead of rendering as a