feat(ai): support InitializeResult.instructions on McpServer

[richburdon]

Summary

Add an optional instructions field to the McpServer layer options
(layer, layerStdio, layerHttp, layerHttpRouter). When provided, the
string is returned in the InitializeResult.instructions field per the
MCP spec.

InitializeResult.instructions is the built-in MCP mechanism for a server
to inject system-prompt-level guidance into the client at session start.
The current @effect/ai API doesn't expose it, forcing downstream users
to either fork or post-process JSON-RPC responses to add it. The schema in
McpSchema.InitializeResult already supports the field — this PR just
plumbs it through the public layer constructors.

The option is purely additive and omitted from the response when not set
(no instructions: undefined is emitted).

Before

McpServer.layerHttp({
  name: "Demo Server",
  version: "1.0.0",
  path: "/mcp"
})

After

McpServer.layerHttp({
  name: "Demo Server",
  version: "1.0.0",
  path: "/mcp",
  instructions: "Always greet the user with a friendly hello."
})

Test plan

• pnpm check (tsc strict) passes
• pnpm test passes — added two tests in packages/ai/ai/test/McpServer.test.ts:
  • asserts InitializeResult.instructions === "be helpful" when the option is set
  • asserts the field is absent from the response when the option is omitted
• Added changeset (@effect/ai minor — additive API)

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 242c342

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@effect/ai	Minor
@effect/ai-amazon-bedrock	Major
@effect/ai-anthropic	Major
@effect/ai-google	Major
@effect/ai-openai	Major
@effect/ai-openrouter	Major

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