feat(mcp): add @modelcontextprotocol/server-sequential-thinking as a builtin MCP

[YOMXXX]

Summary

Adds @modelcontextprotocol/server-sequential-thinking to the builtin MCP set as sequential_thinking. The server exposes one tool — sequentialthinking — that lets a model externalise a multi-step plan and revise individual steps without burning thinking-budget tokens.

Why

Most useful for the small no-thinking-mode models on omo's fallback chains (gpt-5-nano, minimax-m2.7, kimi-k2.x etc.). Without thinking-mode support, those models have nowhere to stash partial reasoning between tool calls and tend to either skip planning or hallucinate it back from scratch each turn. The official sequential-thinking server gives them an explicit scratchpad that survives tool round-trips.

Follows the same wiring pattern as the existing server-memory PR (#4286).

Changes

• src/mcp/server-sequential-thinking.ts — thin factory wrapping the stateless server behind npx -y @modelcontextprotocol/server-sequential-thinking. Returns enabled: false when npx is missing so we never schedule a launch that would die on the first request.
• src/mcp/types.ts — extend McpNameSchema with sequential_thinking so disabled_mcps: [\"sequential_thinking\"] is recognised as a known builtin name (not just a free-form string).
• src/mcp/index.ts — register it in createBuiltinMcps, behind the same disabled_mcps gate the other builtins use.
• src/cli/doctor/checks/tools-mcp.ts — include it in BUILTIN_MCP_SERVERS so omo doctor does not flag it as an unknown external MCP.
• Tests — server-sequential-thinking.test.ts (4 cases: happy path, missing-npx fallback, stateless-environment invariant, default-resolver branch) and an extension of zauc-mocks-mcp-index/index.test.ts to mock the new factory and lock the registration through every createBuiltinMcps scenario.
• Docs — docs/reference/configuration.md and src/mcp/AGENTS.md list the new builtin alongside the others.

Test plan

• bun test src/mcp — 40 pass, 0 fail
• bun test src/mcp src/cli/doctor src/config — 222 pass, 0 fail across 32 files
• Manual: disabled_mcps: [\"sequential_thinking\"] removes it from createBuiltinMcps output without touching the other builtins.
• Manual: missing-npx environments produce { enabled: false } rather than a broken launch attempt.

Notes

The server is stateless — no env vars, no on-disk state, no per-project setup. If the package isn't already cached, npx -y warms it up on first request. Users who don't want the auto-launch can opt out with disabled_mcps: [\"sequential_thinking\"].

If both this PR and #4286 (server-memory) land, the builtin MCP roster grows from 5 to 7. No conflict between the two diffs — they touch disjoint files except src/mcp/types.ts and src/mcp/index.ts, both of which are append-only additions.

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Summary by cubic

Adds @modelcontextprotocol/server-sequential-thinking as a built-in MCP named sequential_thinking to give models a scratchpad for multi-step plans via the sequentialthinking tool. This helps small no-thinking-mode models persist plans across tool calls.

• New Features
  • Launches the stateless server via npx -y; auto-disables when npx isn’t available.
  • Registered in createBuiltinMcps; McpNameSchema extended; omo doctor recognizes it; docs updated. Opt out with disabled_mcps: ["sequential_thinking"].
  • Tests cover launch, missing npx, stateless env, and default resolver; registration tests updated.

^{Written for commit 4217e04. Summary will update on new commits. Review in cubic}

[KennyDizi]

I don't think this is a good idea, the MCP @modelcontextprotocol/server-sequential-thinking is already deprecated.

[YOMXXX]

Thanks for raising the concern! I dug into it before opening this PR and the package looks actively maintained rather than deprecated:

• npm latest: 2025.12.18 (published 2025-12-18, metadata still updated 2026-02-06).
• Source still lives in the main modelcontextprotocol/servers repo under src/sequentialthinking/ — it was not moved to modelcontextprotocol/servers-archived (the path returns 404 there).
• Same install pattern (npx -y @modelcontextprotocol/server-sequential-thinking) is still documented in the upstream README for Claude Desktop, VS Code, and Codex CLI.

If you have a link or thread showing it's deprecated (e.g. a maintainer announcement, an issue I missed, or a successor package), I'd happily switch this PR over to the recommended replacement. Otherwise this aligns with how PR #4286 added @modelcontextprotocol/server-memory — same registry, same publisher, same install shape.