MCP 2026-07-28: wire tests + docs updates for the new surface

[bobakemamian]

Part of #265 — MCP 2026-07-28 compliance. Testing & docs (closes out the migration).

Tasks

• Extend internal/mcpserver/server_test.go wire tests:
  • modern-era _meta round-trip (protocolVersion + clientCapabilities)
  • error paths: -32022 unsupported version, -32602 missing _meta / unknown tool / invalid cursor, -32600 structurally invalid request
  • mid-press cancellation and cancel-after-complete race
  • dual-era coexistence on one connection; tools/* works with no initialize
  • server/discover result shape (resultType, supportedVersions, ttlMs/cacheScope, serverInfo _meta)
• Update the CLAUDE.md "MCP Server (Phase 2)" section for the 2026-07-28 surface
• Update docs/buttons/mcp.mdx; regenerate docs/cli/buttons_mcp.md if flags change

[coderabbitai]

🔗 Related PRs

#193 - feat(serve): buttons serve — REST API over HTTP (#270) [merged]
#222 - feat: auto-update installed buttons [merged]

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