feat: support "draft" as a first-class spec version target (#255)

* feat: support "draft" as a first-class spec version target

- Add LATEST_SPEC_VERSION and DATED_SPEC_VERSIONS constants so the next
  spec release is a one-line change in types.ts.
- Accept "draft" as a valid protocolVersion in the initialize check and
  mock-server response.
- --spec-version draft now selects latest-dated scenarios plus draft-tagged
  ones, so SEP authors can run the full suite against an SDK tracking the
  in-progress spec without retagging core scenarios.
- Forward --spec-version to the client process via
  MCP_CONFORMANCE_SPEC_VERSION so SDK examples can pick the matching
  protocolVersion.

Closes #253

* review: collapse SpecVersion to wire strings; dedup NEGOTIABLE_PROTOCOL_VERSIONS; split out 'extension'

Addresses review feedback from @felixweinberger and @mikekistler:

- SpecVersion is now always a wire protocolVersion string
  (DatedSpecVersion | typeof DRAFT_PROTOCOL_VERSION). The separate
  'draft' tag literal is gone from the type system; 'draft' survives
  only as a CLI input alias in resolveSpecVersion. Removes the
  tag-vs-wire confusion and the specVersionToProtocolVersion mapping.
- NEGOTIABLE_PROTOCOL_VERSIONS in types.ts is the single source for
  what the mock server accepts on initialize (was duplicated in
  checks/client.ts and scenarios/client/initialize.ts).
- 'extension' moved out of SpecVersion into a separate ScenarioSpecTag
  type. --spec-version extension is no longer valid; extension scenarios
  remain reachable via --suite extensions.
- Draft-tagged scenarios now use the DRAFT_PROTOCOL_VERSION constant
  so a draft revision bump is a one-line change in types.ts.

src/tier-check/output.ts

@@ -1,4 +1,5 @@
 import { TierScorecard, CheckStatus, ConformanceResult } from './types';
+import { DATED_SPEC_VERSIONS, DRAFT_PROTOCOL_VERSION } from '../types';
 
 const COLORS = {
   RESET: '\x1b[0m',
@@ -23,9 +24,9 @@ function statusIcon(status: CheckStatus): string {
   }
 }
 
-const TIER_SPEC_VERSIONS = ['2025-03-26', '2025-06-18', '2025-11-25'] as const;
+const TIER_SPEC_VERSIONS = DATED_SPEC_VERSIONS;
 
-const INFO_SPEC_VERSIONS = ['draft', 'extension'] as const;
+const INFO_SPEC_VERSIONS = [DRAFT_PROTOCOL_VERSION, 'extension'] as const;
 
 type Cell = { passed: number; total: number };