feat: migrate transport to the SDK's high-level McpServer API

[sharpTrick]

What

The low-level Server class is deprecated in @modelcontextprotocol/sdk v1
(@deprecated Use McpServer instead… Only use Server for advanced use cases).
This migrates the transport layer to the SDK's official high-level McpServer —
same package, its blessed surface. McpServer wraps a low-level Server
(this.server = new Server(info, options)) and exposes it as .server "for
advanced usage (like sending notifications)" — exactly what Parley's claude/channel
push path needs.

Changes

• tools.ts — register the four reactive tools via mcp.registerTool with Zod
  input shapes as the single source of truth, replacing the hand-authored JSON Schema
  • the separate (already-drifted) Zod parsers + the manual ListTools/CallTool
    dispatcher. topicSchema yields a z.enum for a closed allowlist (advertised as a
    JSON-Schema enum, unchanged) or z.string() when a post pattern widens the set.
• stdio-bridge.ts / http.ts — construct McpServer; the constructor forwards
  capabilities (incl. experimental: { 'claude/channel': {} }) + instructions to the
  underlying Server. Verified registerCapabilities merges (deep, per-key), so
  registerTool's tools.listChanged does not clobber experimental.
• channel-emit.ts — emit the custom notifications/claude/channel via
  mcp.server.notification (the sanctioned low-level escape hatch); same wire bytes.
• push-loop.ts — retyped to McpServer.

Behavior

Preserved: unknown-tool / invalid-input / thrown-handler all surface as isError
results (McpServer funnels them through createToolError, matching the old manual
dispatcher). Also closes a latent drift — limit was int().positive() in Zod but a
bare number on the wire; now one Zod shape drives both.

One visible shift: under a closed allowlist, a disallowed topic is now rejected at
the schema layer (Invalid enum value, which lists the valid topics) instead of by
allow.assert's "topic not allowed". Equivalent enforcement (the enum is
allow.topics()), just earlier and more informative. With a post pattern the topic is
z.string() and allow.assert still produces "topic not allowed". Two assertions updated.

Pre-1.0 note

Changes exported signatures (registerTools / buildReactiveServer / emitChannel /
startPushLoop now take McpServer; ParleyBridge.server: McpServer). Per
CONTRIBUTING.md, landed as feat: rather than a breaking change.

Verification

• tsc -b (build + typecheck, all packages): clean — no downstream plugin touched.
• bridge-core: 124 passed / 1 skipped; transport subset (real Server↔Client InMemory
  path, push loop, dynamic-enum advertisement): 33 passed.
• fakechat loopback (buildBridge → attach → live <channel> push + claude/channel
  capability advertised): 5 passed.
• Grep guard: zero residual @modelcontextprotocol/sdk/server/index.js imports.

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