refactor(agent): nest tool-set and MCP under agent subpaths

[LukasParke]

Summary

• Moves the tool-set implementation into the canonical @openrouter/agent/tool-set subpath and removes the unpublished standalone @openrouter/agent-tool-set package.
• Moves the full MCP implementation into @openrouter/agent/mcp, with nested /create-mcp-tools, /types, /schema, and /cache exports.
• Keeps @modelcontextprotocol/sdk as an optional peer of @openrouter/agent, so base agent installs do not pull MCP dependencies.
• Converts the published @openrouter/mcp package into a thin compatibility facade over the canonical agent subpaths.

GitHub native stack #103: position 2 of 2, above PR #31. GitHub tracks the ordered stack with main as its ultimate base; this PR targets toolkits and contains only 61cca6a and d6b0253 above the parent.

API example

import { callModel, OpenRouter } from '@openrouter/agent';
import { createMCPTools } from '@openrouter/agent/mcp';
import { createToolSet } from '@openrouter/agent/tool-set';

const mcp = await createMCPTools({ url: 'https://mcp.example.com/mcp' });
const tools = createToolSet({ tools: mcp.tools }).resolve();
const result = callModel(new OpenRouter(), {
  model: 'openai/gpt-4o-mini',
  input: 'Use the remote tools.',
  ...tools.callModel,
});

Consumers of /mcp install the optional peer alongside agent:

pnpm add @openrouter/agent @modelcontextprotocol/sdk

Existing @openrouter/mcp root and nested imports continue to work.

Package size / tree shaking

• @openrouter/agent remains sideEffects: false.
• The root and /tool-set entries do not statically import MCP modules.
• A base packed-agent consumer imported root + /tool-set without installing the MCP SDK.
• Packed size: agent ~293 KB compressed / 1.26 MB unpacked; compatibility facade ~4 KB compressed.

Verification

• pnpm build
• pnpm lint
• pnpm typecheck
• pnpm test
• pnpm changeset status
• Agent + facade tarball inspection
• Temporary base consumer: root + /tool-set, no MCP SDK installed
• Temporary MCP consumer: /mcp with optional peer installed
• Compatibility consumer: @openrouter/mcp root + all legacy nested exports

🤖 Generated with Claude Code

Package-boundary hardening

• Loads @modelcontextprotocol/sdk lazily and reports an actionable MCPMissingPeerDependencyError only when an MCP connection actually needs a missing peer.
• Marks @openrouter/mcp and its nested paths as migration facades while preserving every existing import.
• Documents the complete public subpath surface and adds explicit Sentrux boundaries preventing core agent modules from importing MCP.
• Adds pnpm verify:packages, which packs both packages, validates every export target and tarball allowlist, installs them into a clean consumer, checks optional-peer isolation, and rejects duplicate/version-skewed agent resolution.
• Fixes the dynamic FilterToolsByIds<readonly Tool[]> fallback so widened tool IDs do not collapse to never[].

Verification

• pnpm build
• pnpm lint
• pnpm typecheck
• pnpm test — 779 agent tests and 17 facade tests
• pnpm verify:packages
• sentrux gate
• git diff --check

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